Have you looked at the suite of measures produced by the WHO i think there are some short ones there.

Also, when I worked in mental health field in 1990s, the US Substance Abuse and Mental Health Agency, developed one or more Quality of Life assessment instruments. These focused on persons with serious mental health issues, as I recall first developed for adults. All would be free. Might have questions useful for youth in general. And because roughly 20% of youth have one or another mental health problem (5-6% as I recall with what was then labeled 'serious emotional disturbance') and because currently youth suicide rates are rising dramatically, at least in US and probably elsewhere, in part due to the global environmental crisis, rising racism and authoritarian reactionary regimes around the planet who offer nothing to cope with these crises, a result of widespread youth sense of 'hopelessness', I recommend you include quality of life questions to address the mental health domain.

Whoqol Bref é um instrumento que avalia a qualidade de vida e tem 26 itens. http://apps.who.int/iris/handle/10665/63529

I use the WHOQOL-BREF generic scale 26 items, normed with mixed-age population. There are some requirements to use the scale (permission, contract etc). It is free of charge, though. Please, see http://apps.who.int/iris/handle/10665/77932

The scale is internationally rooted and can be found in many languages. I would recommend this one. There is also another one, which I know quite well, although not apply in my research: Personal Well-being Index (PWI)(short version) (by Rober Cummins; Deakin University) internationally used, free of charge. Needless to say PW is conceptually related to QOL (need to be careful there....)

Best

Lidia