QUOTE(Invidia @ Oct 21 2010, 01:42 PM)
I did music A level and it was easily the most enjoyable college class I had.
What it involves depends on the specification, but I'm guessing they're all relatively standard in that you have set works to study for a written exam, then you get an aural based written exam then on top of that you have to do some performing and composing. Oh, and harmony; all A level music syllabuses will probably have a Bach chorale or counterpoint or something like that.
In terms of a career in music, it depends. Some universities will consider you if you can prove you are at a level equal to or beyond A level music, say with grade 8 theory and practical or something. Conservatoires don't really care about A levels; they all say a pass in music plus two other subjects which is an E because they are audition based so they can see how you play and a few of them give you listening/theory exams after your audition so they assess your level that way.
Teaching, if you want to do a diploma you just need grade 6 theory plus 8+ practical (then grade 7 theory for LRSM and grade 8 for FRSM). PGCE you need a degree; I am applying for that at the moment so can guarantee that without a music degree you will not get onto PGCE music teaching; it's very competitive these days.
So ultimately, if you are looking at instrumental teaching you don't *need* A level music or a degree, it will just make you stand out more to potential students to say you have those qualifications; non-musical people have no idea what a teaching diploma actually means, they understand degree =P
Classroom/school teaching, yes you do need A level and then you need to go get your degree.
Just to clarify that you need theory if you want to do a teaching diploma, but not for performance
I know that the OP sounded like she might want to do a teaching diploma, but just thought I'd point this out.