I've found I have to play with more restrictions but otherwise, it's ok. I would usually rotate when I got bored or fancied a change, I've never had a set schedule. Now I make sure I finish a term with whoever I am playing with. I have a group of about 6 YA's I'm rotating through, I just had the first one graduate. I've been playing with two sims in each 'family'. When they finish a term, I move them back 'home' to their parents, and imagine they're on summer break, and pick up the next two for their next term. So far everyone has been getting good grades. I would really prefer it if I could have them all enrolled at once, and rotate round without anyone continuing their terms without me, so they would at least all be enrolled together and see each other around Britechester. I've gotten over this by having the rest of the rotation in a club and set the pub in Britechester as their hangout, so I just head there if I want to socialise with them.
