I went for a brush up at CATS three months before I went for the exams so it could fit in with my holidays, there were no problems with that. There was also a guy on the course who was in the forces and took the subjects in an order that suited the exam days. i.e he studied all the subjects that had Tuesday exams first in order to only have to take a day off instead of whole week each time.
I think most schools will be flexible as long as you show you are committed and do all the required work in advance. Certainly I found CATS very helpful and flexible in this regard.