Hi,
If you are reasonably intelligent you'll be fine.
This was my classes final A Level grades:
1- A* (me lol)
1- A
2- B
5- C
1- D
This was from a bad comprehensive.
My class went from A/B's at AS to B's/C's at A2. Because we did political ideologies at A2, which is tricker than US government which most do at A2. All pretty much do UK government/politics at AS.
So you should look into what modules your school offers. But me and my good friend got A* and A respectively and we were really the only fairly smart ones in the class. So you'll get an A/A* if you're somewhat clever, it isn't that hard.