It depends on the school that your planning on going to, some schools need you to have certain grades on particular subjects, this is strongly the case with english, maths and the sciences, but with drama it might not be as it's not a core subject.
The best way to find out is by checking the school's website or going along to an open day and you'll find out the requirements - at the school I'm at now there was a leaflet with all the info on their website and you could ask the teachers questions on open day.