I would recommend you to check following articles on this topic. 

http://jei.sagepub.com/content/32/3/163.abstract

http://onlinelibrary.wiley.com/doi/10.1002/icd.1826/abstract

http://tec.sagepub.com/content/32/3/132.abstract

First, let me state that I have no personal expertise in this subject.  Secondly, I suspect that most if not all children recognize "different" children and that many discriminate against them.  Third, discrimination against persons who differ from whatever happens to be the local norm is common throughout all life stages.  

You might find the links below a place to start since the first leads to a set of references (the second).  They are not, however, specifically/exclusively about disabilities.

http://www.adl.org/assets/pdf/education-outreach/How-Do-Children-Learn-Prejudice.pdf

http://www.adl.org/assets/pdf/education-outreach/References-for-Question-Corner-Installments.pdf

Children with disability are being treated in a scandalous and embarrassing way. They treat them like trash. Sadly the treatment of mentally disabled person is sadly scandals in some countries

teenink.com/mobile/435017

Yes. It is my personal observation, that usually disabled children were ignored by normal children 

Yes. It is my personal observation, that usually disabled children were ignored by normal children. One reason may be lack of competitiveness due to their disability.

Based on social identity theory and theories of categorization, that could be explained because human being perceives spontaneusly some social categories (gender, race and probably disability is one of these categories too) and we are intrinsecally motivated for positive distinctiveness of our own group, and this is easier to achieve by derogating the outgroup.

This fact could be avoided or we can reduce its influence through diversity training and intercultural education. Children discriminate against disability because they are not educated in valuing social differences between groups, and as you mentioned, they don't know how is the best way to treat people with disabilities.

children usually learn  and recognize the difference between them and those with disabilities  , physical and mental disability is starkly visible and hence the knowledge that they are different . However, Discrimination as says the definition is prejudice in action and  is therefore a learned behaviour  be it vicarious or intentional , it has been acquired in the way the learning has been transferred in socially acceptable/non acceptable ways and culturally determined parameters of the society.

My article Socialization of hearing impaired... (Volta Review posted here at research gate) deals precisely with this topic of discrimination against those with disabilities. In the study we noted that at the top of the hearing children in the day camp was a leader, Alpha- he was more aggressive and nasty than the others but he had followers 'subjects' who vied with each other to be his 'best' mate. The Alpha would pick on and be mean to others he felt were weak and/or different and his subjects would emulate this behaviour. The weak ones were the first to befriend the hearing impaired children but none of the 'subjects attempted any positive interaction while their leader rejected them. The hearing impaired children were very naive and they felt that everyone loves them- goes to show that they were lacking the reality check skill to verify their social status in the group. My conclusion was that in order to  eliminate this, educational systems must integrate all those with disabilities into regular classrooms from the start and actively hold regular discussions starting in kindergarten and continued into end of high school about acceptance and tolerance. This would be a metacognitive approach with a 'metasocial goal' that would impact all children. I was so reminded of Orwell's Animal Farm while I was collecting data for my study. 

Because of difficulties with social interaction and the inability to read social cues, children with disabilities have higher rates of peer rejection

Students with developmental disabilities may have difficulty paying attention to more than one piece of information, which may cause them to stay “stuck” in a conversation. Such actions can have adverse effects on their social skills and make it difficult for them to hold conversations and make friends. Children with communication disabilities often have assistive technology devices that other students do

not understand and, as such, the other students view them as “weird.”  

Dear David,

Attitudes towards children with  disabilities  by those without disabilities are shaped primarily in family houses. In  Polish public schools, where there are integration/inclusive units, it happens that parents of children without disabilities do not want their children to learn with those with disabilities in joint classes. They are afraid that learning in the integration classroom reduces the possibilities and level of education for their own "abled" children. Such distant attitudes are chain reaction for the future subsequent devaluation, discrimination against people with disabilities.

I put a link to my text about Polish lights and shadow aspects of integration.

Warm Regards, Beata

https://www.researchgate.net/publication/264311761_International_Research_Journal_for_Quality_in_Education_A_review_of_lights_and_shadows_of_Polish_educational_integration 

Article A review of lights and shadows of Polish educational integration

The key reason, though there are many, is a lack of personal relationship.  We are by our very nature a class driven society.  It is easy to subjugate those we don't know or understand.  The way around this problem is proximity, intimacy, and modelling by caring and understanding adults.  The answer is not isolating those with disabilities which amounts to blaming the victim.

The original question can be answered with an opinion rather than a reference to research, I joined the discussion and have been following it in hopes of learning about evidence related to the question.  For that reason, I know that I have found the answers from Ibrahim Acar, Yvette Hus, and Beata Borowska-Beszta particularly helpful and how that others may also be able to contribute research-based answers.

Dear Beata, we have the same problem here with 'some' parents blaming integration for all the 'ills' of their children's education. Fortunately, we have strong laws that protect the right of every child to attend regular public education AND get support so that the school is held responsible for the progress of the child with the disability. I heard a psychologist claim that if the disabled child gets extra time for exams, ALL children should get extra time- she resented even that small accommodation that is given to children with learning disabilities. So Tragic! It is up to us who care and know to help change attitudes in the general public. 

 Dear Yvette, 

I'm aware that we in Poland need more comprehensive  education in general public to do, to raise a good level of acceptance of our citizen with disabilities. I notice that the best things of the acceptance happen in niches and disability cultures but not in the mainstream and dominant culture. This general public education will take years ! Even If we signed in Poland many world/EU rights/laws concerned disability rights, the implementation in some spheres through life-cycle is still on poor level. I mean even if children/youth receive proper education, they have poor opportunities as adults. For e.g. accessibility to valuable, paid employment, independent living especially for adults with intellectual disabilities. 

Beata thanks for responding. I agree that it takes years and lots of effort to change attitudes and realize that giving ALL citizens the right education and opportunities great benefits everyone. I'll resort to Chinese philosophy (Laozi) here: A journey of 1000 miles begins with one step. Best of luck in your efforts to change things for the better. 

 Yes, Yvette, Laozi is right and I agree this road is long, and I hope promising.

I believe a certain part of the world needs more awareness concerning disability , we in India do not have any facilities/schemes introduced for the mentally challenged yet .They are still considered a liability not only in the families they are born to but also by the society . Acceptance ,empathy and care is lacking even at the government level

Dear Vineeta

I see the experience in some countries is scarce. Today there is no adequate training and qualification. Children face alone the relationship with children with disabilities and this is a problem because they avoid the interaction to avoid failure.

Thank you all for the interesting contributions

I'd like to know studies about this reality

Dear Ibraim

Thank hoy very much for your reference

Best reguards

Dear Stephen

Thank hoy very much for your references

Best reguards

 Dear Ana, Vinetta, Archana

Thank you very much for your interesting opinions

Best reguards