Thanks for all the replies.
I think DS1's problem is that he's struggling to adjust to how busy he is. I don't think he is exceptionally busy - chances are your DC are all equally, if not more, busy - but DS is unhappy about it.
The school has a lot of lessons and activities outside of A level classes and a lot of them are mandatory. So there are very only two free periods per week and he is often expected to give up his lunchtimes and stay after school for nonA-level stuff. On top of that, he has a longish commute (about an hour - so,again, nothing worse than many of your DC will have). Then he has approx 15 hours or homework per week.
The upshot is that he is busy from when he gets up at 6:45am, until about 9 or 10 pm every day, plus usually a day and a half at weekends.
Somehow its all getting on top of him and he is losing his concentration when working. He knows how to do the work, can easily do it when relaxed and hasn't come up against a topic yet that he hasn't easily understood, but he's making mistakes all over the place in his work.
Unfortunately, i think the school will try to help, but they will think the issue is that he is struggling to understand the concepts and make his days that bit longer by putting him in catchup classes.
I don't want to interfere, because I don't think i can, but I know Ds will not try to speak up for himself at school, and he'll just try to take on the extra burden, when the real problem is that he just needs space to centre himself. DH calls is "resilience".
Any advice?