Yup, for the older students, wearing the winter uniform is an obligation. Most Japanese schools have a set calendar for their uniforms, and the new school year starts in April, which is still officially part of the "winter uniform" season that usually runs until the end of May. So this is just a special exception made for the new students in some Okinawan schools because they'll only be wearing their winter uniforms for 2 months or so until the switch. Hope that helps clear things up!