Motivation, and especially autonomously regulated motivation, is considered to be an important factor influencing people’s participation in sport, to which children and adolescents are attributed a special significance with regard to their development. With reference to a decreasing participation in sport in adolescence—at least in formal club-based sport—the question is raised to the significance of the trainer’s behaviour in relation to motivation in the school sport context. The latter is also attributed to influence recreational sport. 187 club athletes aged 14.1 years (SD = 2.33) were asked about their autonomous motivation in school and leisure sport, about the autonomy-supporting behaviour of coaches and about the intention to play sport or to drop out. Regression analysis show significant (p < 0.05) relations between coach behaviour (β = 0.23), school (β = 0.33) and leisure sport-related motivation. This, in turn, is significantly related to the intention to participate in sport (r = 0.26) and to dropout (r = −0.23). School sport-related motivation has a higher explanatory power than coach behaviour, which proves the relevance of school sport promoting motivation for leisure sport activity. Decreasing motivation for school sport thus represents a pedagogical challenge regarding lifelong sporting activity.