Do I have to do a fourth AS?

I got my GCSE results just over a week ago and I'm happy with my results.

For AS Level I'm planning to take English Literature, History, French and Philosophy.

I did an AS in Critical Thinking alongside my GCSEs and got an A.

I have various commitments: work, volunteering in a charity shop, DofE, singing exams and socialising.

I am worried that I won't be able to do all this and achieve the grades I want.

Is my school allowed to make me do a fourth AS or could I opt out of Philosophy?

Would I be making a mistake?

And yes, I know that CT isn't a respected fourth subject :P