oh god oh shit ok here goes:

sora - i think we all need to come to terms with the fact that, realistically, sora WOULD be an RSA student. but for funsies, he'd be in savanaclaw
riku - diasomnia. a dorm for students who excel at magic, and riku can wield darkness like no one else.
kairi - so we all know girl's aren't allowed in magic school for some reason BUT i'd have to also stick her in savanaclaw

roxas - ignihyde. spirit of diligence and also he lived a computer for a while :)
xion - pomefiore. "spirit of tenacity" and ALSO there's definitely a Unique Style in having several different peoples' faces.

ven - heartslabyul. kid is used to the strictness of master eraqus, he can handle the queen's rules. also he'd look SO CUTE in the dorm uniform
aqua - octavinelle. water dorm....aqua...ha (but also the benevolence and intelligence that comes from students in that dorm.)
terra - savanaclaw, duh.

namine - scarabia. mindfulness, careful deliberation and planning, etc. she also would look cute in the uniform
vanitas - also scarabia because nightmare vanitas kicked my ass in bbs and someone with that kind of fighting skill ABSOLUTELY has planning skills