Oh therereallyis I could have written your op. Not verbatim, my school are putting me through my own shit in their own special way.
However, the bit about throwing yourself down the stairs? Yes, that's me too. Right now, today. Although I haven't been considering the stairs route, I've been thinking a lot about driving into a tree or similar.
Although I have a mortgage to pay I know I have to get out and I strongly suggest that you do the same.
I have no idea what I'm going to do instead but I've handed my notice in for 31/8. Even if I have to do supply someone, somewhere will employ me.
In the meantime PPs have offered good advice, I strongly suggest calling one of the helplines, that's what I will be doing later.
I hope you are doing ok, you have my empathy, sympathy and best wishes.
Sadly, you are not alone, please look after yourself 