This longitudinal study covering two educational transitions examined 1,821 Finnish students’ participation in and identification with school and their associations with students’ academic achievement and truancy. The students were surveyed (a) at the end of primary school, (b) at the beginning of lower secondary school, (c) at the end of lower secondary school, and (d) in the first year of upper secondary education. In alignment with the participation-identification model, higher levels of participation in school activities at the end of primary school predicted higher levels of identification (i.e., feelings of belonging and valuing school) at the end of lower secondary school. This association was mediated by academic achievement. High levels of both participation and identification at the end of lower secondary school predicted lower levels of truancy in upper secondary education. The study indicates that promoting students’ participation and identification during comprehensive school prevents student disengagement from upper secondary education.