Split And Move (to same Dorm) Not Working

@LunarCynthionThe dorms are not like TS3 or TS2 all sims that join dorms are playable sims. All sims that join the dorm from the school window are playable sims, so you must play all playable sims in the dorm. If you want sims to go to school with your sim create a club and call it dormmates or whatever, than go into edit dorm and choose the club dormmates, they will than show up as roommates if they are old enough to qualify.

Edit 1: think I miss understood your question, I think there is a protection in place if there is kids or pets when you move the last member of your family into a dorm your have to sale your house, this game considers university a full move instead of a temporary move.  If you have objects in your family inventory the game won't allow last members of the family to move out  to dorm, as dorms do not have family inventories.

Edit 2:  In order for all family members to move out into the same dorms they must all be accepted into the same University.