Gender homophily represents one of the most persistent patterns in friendship formation, yet its sensitivity to classroom gender composition remains understudied. Using longitudinal friendship networks from 708 classrooms in the CILS4EU dataset, we employ Bayesian multilevel Stochastic Actor-Oriented Models (SAOM) to examine whether and to what extent classroom gender composition moderates friendship preferences during adolescence. We test whether the shifting salience of gender boundaries across the compositional spectrum activates in-group identification and out-group avoidance, thereby shaping same- versus cross-gender preferences. We find that the relative preference for same- versus cross-gender friends remains largely rigid and unresponsive to compositional variation. However, post-estimation analyses provide modest evidence that same-gender preferences strengthen relative to cross-gender preferences in girl-minority contexts, and mainly among the girl minority. This asymmetry suggests that the experience of numerical minority status intersects with gender hierarchies. These findings demonstrate how cultural scripts constrain adolescent friendship selection, suggesting that gender boundaries are largely resistant to compositional variation, and where composition does matter, it reinforces rather than diminishes gender homophily.