Social networks have an important effect on health, and social network analysis has become essential for understanding human behavior and vulnerability. Using exponential random graph models (ERGM), this study explores the associations between mental health and network structure (or more specifically, mental health homophily) and the association between poor mental health and social isolation. Two classes of Romanian adolescents aged 12-14 years participated in the study (n = 26 in each class). We assessed school network, sociodemographic covariates, and mental health using the Strengths and Difficulties Questionnaire (SDQ). ERGM was first used to test the presence of sex and mental health homophily and then to test whether mental health was a predictor of social isolation. The results showed homophily patterns regarding sex and mental health. Moreover, participants with a higher SDQ score had a lower probability of a tie. Overall, this study showed how social networks are structured with different forms of homophily and that adolescents with poor mental health are more likely to be social isolates. Thus, prevention and interventions should focus on these vulnerable adolescents. Methodological advances like ERGM constitute a promising avenue for further research.