Poor school-related well-being may influence adolescents’ school performance and lifestyle. Adolescents having disabilities or ADHD are in a vulnerable situation for having poor school-related well-being, compared to adolescents not having disabilities. We used cross-sectional data from a school-based survey among 15–18-year-olds (N = 4071) in Sörmland, Sweden, to analyse the association between poor school-related well-being and disabilities or ADHD. The analyses were carried out by logistic regression models, adjusting for background factors, school-related factors, and health-compromising behaviours. Adolescents having disabilities (n = 827) or ADHD (n = 146) reported that their disability had a negative influence on school. Compared to peers without disability, those having disabilities had an increased chance (OR = 1.40 95% CI: 1.17–1.68) of poor school-related well-being. The corresponding OR was doubled for adolescents reporting ADHD (2.23 95% CI: 1.56–3.18). For the ADHD group, the adjOR for poor school-related well-being remained significant (1.67 95% CI: 1.13–2.50) after adjustments for school-related factors and health-compromising behaviours, but not for the disability group. In conclusion, adolescents having ADHD are a particularly vulnerable group at school, having a greater risk of poor school-related well-being. Schools should actively work to achieve school satisfaction for adolescents having disabilities, to ensure that all students have similar opportunities for favourable development, health and achievement of their academic goals.