We "retrain" the aides at the start of every school year. Actually, we go in a few days before school starts and work with them. We show them how best to help Justin move forward a little to get his pants down and how to use the hoyer lift. Before they started using the lift, we showed them the best way to pick him up and move him. If your son doesnt use a handheld urinal at school, I would recommend it. Its much much easier than the toilet once they are in the chair full time. Also, we put Justin in a size or two larger pants and a size larger underwear to make pulling them down even easier, and we try to put him in the breakaway pants and shorts(the ones with buttons on the side) for even more ease.
My mother finds the breakaway pants alot at Academy sports. you can find them on clearance alot and at their online store. Our school has a hoyer lift, but its our hoyer lift. We take it up there and let them use it. We have a multibase lift we use at home.
My son is also 12 and we are having the same issues. The school nurse was originally trying to transfer him, but it was too much for her and she couldn't do it by herself. There is currently a male aid that goes between two schools that helps get Jason to the bathroom. We tried a handheld urinal a few times at the end of last school year, but Jason would end up with his clothes wet, so we thought the transfer method from the chair to the toilet would be better, until he gets too big to handle. I do try to put elastic waist pants on him that are a little too big in the waist to make it easier to pull them up and down. We may have to try the urinal again soon to see if that is any easier now.