I had a real problem with bachache when I was in the sixth form and as i was doing 2 science A levels I spent at least half my lessons in a lab. The school supplied me with a wooden stool with a back, just like the normal tall wooden stools used in labs but with a back, and that was 30 years ago! I would have a look on the internet to see what is available, and ask at school to see if they either have something or can get something for you to use. The other thing that you might look at is a perching stool, but with that you are not actually sitting, but for some people they work well, others don't get on with them.
Some office type chairs can be adjusted to give extra height, so this may be another option. Do explain to someone the problems you are having and how it is interfering with your studies.
Diagnosed HEDS December 1st 2005. DD1 (20) HEDS and scoliosis (now corrected by surgery), diagnosed June 2006. DD2 (18) mild HMS. Son (11) some hypermobile joints, poor muscle strength and seems to be developing scoliosis as well, woopee!