Schools are increasingly concerned with the well-being of the whole child - likely, more so since the COVID-19 pandemic - and goals here were to document the psychometric properties of a brief new measure of adolescent mental health, the Well-Being Index (WBI). The measure assesses 4 symptom areas, 2 each of internalizing and externalizing symptoms-Depression, Anxiety, Rule-Breaking, and Substance Use-and an optional scale on Isolation at School. A total of 2,444 students from 2 high schools completed the WBI, the Youth Self-Report (YSR), and other related measures. Alpha coefficients showed acceptable internal consistency, with values for the 5 WBI subscales at .83, .84, .78, .79, and .74, respectively. Both exploratory and confirmatory factor analyses demonstrated consistent factorial validity. Correlations with corresponding YSR subscales indicated good convergent and discriminant validity. The WBI Substance Use and Isolation at School subscales, similarly, had high correlations with subscales from preexisting measures. Criterion-related validity was indicated in significant correlations between WBI subscales and conceptually related dimensions of close relationships. Also examined was the percentage of youth falling above clinical cutoffs on both the WBI and YSR, and findings demonstrated high concurrent validity. Collectively, results suggest the promise of the WBI as a brief, psychometrically sound measure to assess the adjustment of adolescents, along with perceptions of school climate that can be modified toward fostering their overall well-being. (PsycInfo Database Record (c) 2020 APA, all rights reserved).