Speaking as a teacher, and someone who has been involved with contract disputes as a union rep I would give your options careful consideration, this is how I see it...
1/. you go off sick, upside is you get out, downside is you potentially have 4 months of sick absence on your record, caveat is many non teaching employers understand the stress aspect of teaching. It will however make any return to teaching difficult.
2/. You give notice, leave at Christmas, this is the best and cleanest option, its entirely above board, no bad feeling, you can go back to teaching if you ever wanted to and will get a good reference, downside is unless you have found something, your pay stops at Christmas.
3/. Quit with immediate effect, a lot is talked about the notice periods, but in reality they are difficult to enforce, leaving early puts you in breach of contract, its highly unlikely they will come after you for money, but can go through a disciplinary process in absentia and bugger up your reference.
4/. You work on, look for a job outside of teaching, when you find one you give the school as much notice as possible, preferably full notice. This quite often happens when people leave teaching for a job outside, and its best to be upfront and negotiate a release from the contract.