I don't see why so many people had such a crap experience? Last year, the year 11 students got mostly A*-C at my school, only 1 or 2 getting a D I believe. We never had to do extra hours at lunch or after school, if you work hard in your lessons it's easy to get the 10-15 minutes of acting that you need.
I think one thing that benefits my school is that our drama teacher used to teach at a performing arts school, yet he is beyond laid back. He knows that by doing loads of extra, stupidly unnecessary work it won't actually get you anywhere so he just helps us do the right amount. The only hard bit, really, is the written part but if you just figure out the structure and spec. you're fine.
Also, even if you're not confident then just go into a group with people you're comfortable with, or go in a smaller group. We aren't forced to do anything in my class, you can choose your group, you can choose your script, you can choose your lighting and all the rest of it. I don't know if it's just me, but my Drama experience is amazing and I wish I had more drama lessons just to have more fun....I look forward to them and think it's taken me away from the shy person I was in year 8/9.