The district court abused its discretion in denying class certification on the ground of lack of numerosity in this Title IX case charging unequal sports programs for females in Hawaii state schools. The absolute size of the class–300 currently enrolled students–satisfied numerosity, as the plaintiff’s claims challenged the state’s overall treatment of female athletes, necessarily affecting all 300 female athletes. The fact that the 300 could be identified didn’t cut against certification since the standard is whether joinder is practicable, not whether it is impossible. Furthermore, as the only relief sought was prospective declaratory and injunctive relief, there would be little benefit to joining the other athletes as parties. Also, the district court improperly overlooked the fact that the putative class included future female athletes. When a class’ membership shifts over time, certification is more appropriate as it is difficult to join a shifting group of plaintiffs. The district court also erred in concluding that the retaliation claim did not affect athletes other than those on the women’s water polo team who complained and were retaliated against. Other athletes could assert the claim since they were within the zone of interests protected against retaliation and may have been deterred from complaining by fear of similar retaliation against themselves.