It is intended to differentiate unaffected individuals who have no or dysfunctional psychopathological frames, assessing the health or quality of life.
I guess it depends on what you mean in terms of "school population" If you're meaning elementary age students I tend to like things like the Behavioral Assessment System for Children-2 or the Child Behavior Checklist (both these instruments have multiple respondent forms so you can get info from child,parent, teacher).
Also, at the risk of being self-promoting I do have an instrument that I've developed that is a parent report for children ages 3-10, which looks at a variety of interpersonal and intrapersonal factors.
Holliman, R. P., & Ray, D. C. (2013). Development of the Child Interpersonal Relationship and Attitudes Assessment (CIRAA). International Journal of Play Therapy, 22(2), 59.
If you're looking at more teens, you might take a look at this page which has several clinical tools available for download in a variety of languages http://teenmentalhealth.org/for-health-professionals/clinical-tools/
We're trying out the HoNOSCA this year as a regular assessment instrument. We needed something short and free that assessed multiple domains and had a self, parent, and teacher version. I am not sure if they have cutoff points for screening though. It's out of Britain, so we had to Americanize some of the wording: http://bjp.rcpsych.org/content/177/1/52.full.pdf
Also, are they any specific instruments that used for low SES populations groups – with regard to measuring psychological wellbeing or mental health –
Further, are they measures that are very easy to complete by students in a school – that is able to capture their mental health and illness
I want something teachers and school staff can use – and that is open and that does not take up a lot of time but that it should be valid and reliable.
The Dessa mini is a great tool to use for screening--very short and easy to use, but with sound psychometrics and technical support and research behind it.. It's developed for screening purposes and if a score indicates a possible weakness, use the full assessment tool for just those students.
You could use the Dominic Interactive (DI) for children aged 6 to 11. It is a computerized multimedia (drawing and voice over for questions) structured questionnaire based on DSM-IV criteria, covering 7 psychiatric syndromes (the most prevalent). It takes 20-25 for children to complete the DI. The psychometric properties have been verified and the validation study of the computerized version has been published recently (Canadian Journal of Psychiatry, August 2013, Bergeron et al.). There is also an adolescent version of the DI, with published psychometric properties (Canadian Journal of Psychiatry, Bergeron, Smolla et al., 2010). I think there is a Spanish version of the DI.
If you are to identify the target group a bit more and give a bit more insight into the purpose of the study with regard to implications - that would be very useful for this conversation
I mean, that CBCL is too long and complex for a screening, it is very good for epidemiological investigations. Very good screening test is the Strenghten and Difficulties Questionnaire. Many countries used and publised data with this, so you can compare your data with them too.