The only way you can do A levels is by paying for it.
You could ask the head teach of the sixthform to agree that you'll pay for your own a levels, in a breach contract that you are obliged to be learning within the school for a levels.
So say for example, you want to do a level chemistry, biology and physics ( one a level costs around 200 pound), you can do the tests at another test centre ( or the test centre at your college if they allow you to) and learn at the sixthform. thats what I would do, because you are obliged to a have a free education in sixthform for 2 years at aged 16-18. so you can learn there methods there and do the exams at another test centre.
There are test centre out there right now which are giving offers to do 3 a levels for 200 pounds, i think thats worth. but thats the only way you are going to get in because now our days, most sixthform ( which i think is stupid) will not allow you to do the course you wana do.