If they wear flip flops for PE, they're likely to injure themselves. Having the correct outfit and equipments for PE is important and if you wear flip flops during PE, you wouldn't be able to move around or do the sports properly. They should wear better footwares for activites for safety and to enjoy the activities.
Flip flops are not correct footware for any sporting activity. They are an accident waiting to happen. It is just another sign that people are not serious about learning anything important at school. Students wanting to excel and compete would be wanting every advantage for their PE. Not being prepared with proper footware for PE is a sign that they are unprepared for their other classes (can't spell, can't use English grammar, can't do math, won't memorize multiplication tables), and also not going to be prepared for life.