during class break, before class, after class - those times you can ask other ranked members to work with you, be your uke or be their uke to get your exercise in. think out of the box (mine is chinese take-out and you never know what you get) for the solution.
when the mat is set, the class starts right away. after the class has ended, the mats get stored in the storage room right away. during class breaks, the new people also asks me to teach them the previous techniques and I have to do the administrative works such as signing their attendance book.. and more often than not, I'm the only senior student to attend the class.