There is a couple of things going on here, its not just incorrect technique.

Firstly what I like to do to isolate my abdominals better when working out is to do inclined sits. I find it a lot more difficult to allow my hip flexors to take over when doing them this way. Hip flexors are down the front and just to the outside of your thigh, and thats where you will feel a burn as well i am gussing? Or does back pain stop you before that. Out of choice I like to do crunchs hanging from a bar, BUT this is obviously a little dangerous as you can land on your head, and also they aren't very keen on you doing it in a gym! But if you have a chinning bar at home it is perfect,a nd you can even go the whole hog and get gravity boots!

Secondly you need to protect your back by increasing muscle size. nobody would work their bicep and not reciprocate on their tricep. Pretty much all muscles have a matching muscle which will do the complete opposite. For your abdominals these are your dorsals. Ideally for every sit-up/crunch you should match it with one dorsal raise. To do this effectively requires a special piece of equipment. But you can do them over the arm of a sofa if somebody holds your legs. Match this with flies to get a really good chest work out at the same time.

Finally mad boxo, you are at school right? Are you in the school cricket team? Do you bowl? Bowling in cricket can be a very lop sided action which can cause back problems (more so with pace bowlers). If this is the case then you should consider seeing an osteopath or chiropractor as they can sort out a lot of issues.

Hope this helps!!!

Chris