Sorry I didn't answer properly, it's been a long day!!
the Med. girl had to do astro with the rest of us as in a class of 6 our teacher wasn't willing to split the group just for one of us (my school is really inflexible like that).
In non-astro practicals (if you were asking that; I'm not sure) you do experiments to prove what you learn about. There is, I'm sure you know, a practical exam which makes up about 20% of your AS mark (for Edexcel at least).
So the class was not split for topic at all. And as I said before there were no practicals for astro as it's sort of impossible, us not being able to go into space!