DS was miserable at school. Eventually we took him out and continued with home tutors; he took the ASs at his old school which we agreed with them before he left. He did OK - not fantastic but certainly better than he would have done at school. His problem was complete lack of interest and motivation so keeping him up with doing work at home was very difficult - you can make them sit in front of their books but making them learn or memorise anything means you have to do it with them 100% like you would a toddler.
Taking him out of school was the right thing to do (especially as it was a private school!). I'm not sure whether shovelling him through his As was right though - he didn't get good enough As to go to university and anyway has always been absolutely firm that he is not going.
We were blinkered - he'd always been in the top set for everything, scholarship entrant, school had him down as an excellent university sciences candidate etc. Unfortunately we didn't pay enough attention to him and to what he wants out of life - which is not a glittering career in the city, but a quiet bedsit, some peace and quiet, a top-quality pc, a very few close friends, and no stress.
My daughter is having extra tuition because she so badly wants to go to University that I want to be sure she gets her grades. Her tutor has a mature (ie 25 year old) student who dropped out of his As at 16 and has come back now. He got an A* at a level after studying with her for just 5 months, not because he is a genius but because he is a bit more mature and really really wanted to do it.
I would say - look at your child. If they are struggling with their A levels there's a reason, and at 16/17 they are old enough to both make a few of their own decisions and to take the consequences of those decisions. There are alternatives to the As/University/Career sausage factory that all kids are pushed through. They are all going to live till they are 100, there's plenty of time and opportunity for all that later on if now's not the time.
I wish I had said to my son 'you can stop your As but if you want to live here it's going to cost you xxx per month, so go and find yourself some work'. Pushing him through his As against his will hasn't achieved anything for him - he stacks shelves at the local supermarket, and still has no ambitions.