My subjects are year 5 pupils(11 years old)  from an elementary school in Malaysia.  There are 60 year 5 pupils in that school and I have selected all the 60 students and randomly assigned them into experimental and control groups. I am still not so sure if I could make generalization to the population in the district just based on the findings from a school?  Shall I add another school or should I change my method to quasi experimental by choosing all the year 5 pupils in that school without randomization?  Thank you so much for helping me. 

The short answer is "probably no", unless you have good evidence that your school represents the district well on all the important characteristics, whatever those are. It would have been best to have randomly sampled 60 students from the district.

"Generalization to the whole population" deals with the concept of external validity. Sampling of subjects is what directly affects external validity. Random assignment affects, at least directly, the internal validity of a study. However, you can't have external validity unless you have internal validity.

Thank you so much

My accessible population is year 5 pupils from a particular zone, and I have focused on 11 years old pupils from a Chinese school.  There are only 2 Chinese schools in the particular zone.  So I have selected a school based on random sampling method. In my opinion,  if I carry out experimental research in a particular school following scientific procedures of experimental study, I can control location treats and other treats as well.  May I know if anyone is in the same view as I am? 

Thank you for your feedback. 

First things first. You are conducting a study and your design is true experimental. It appears to me you want to generalise the findings to the entire district. Right? You have a total of 60 pupils (from one school) whom you have randomly assigned to both the experimental and control groups. It would be impossible to generalise the findings to the district. The sample in the both the experimental and control groups should be REPRESENTATIVE of the entire district population. Please ensure that first. I doubt if that can be achieved from one school. If you have further question please do not hesitate to contact me again. We can engage further for clarity.

I believe you can't, because as all of others colleagues said, it's not representative. Generalizatios in education and with children is not a light matter. What you may say is that there is a TENDENCY in a particular group of students (say chinese scholl, x district, x grade).

Good luck!

Thank you.