A large literature shows that relatively young students perform worse in class. Using data from the 'Health Behaviour in School Aged Children' international survey, we additionally find robust evidence that they are aware of performing poorly, they spend more time watching TV and less time doing sports than older peers, while tending to spend as much time as older peers on their homework. We use a two-stage least square to instrument both relative and absolute age, which turns out to be an important issue. Heterogeneity analyses show that most of these effects reverse or disappear in time.