I pretty much forced my friend to do Bio 3/4 with me because I didn't know if there were other year 11s who would want to (there were so hurrah for that). She thought it would be fun, but ended up struggling a bit. I offered to help, but I think she just gave up after a while. Whoops. Luckily for her she's doing 223094823 other subjects so her Bio mark won't even count.
Anyway, don't feel bad if your friends are lazy and expect you to do their work. I think that's ridiculously selfish and I personally rage whenever I'm expected to do everything in group work just because I'm more competent. It's not like you made them pick their subjects and they're the ones who should've treated their subject selection more seriously. If they're really struggling that much, they could just drop it and pick up something else.
There's a difference between Year 11/12 and wtf at them blaming you and disrupting your learning as well. Just explain the concepts to them, but you shouldn't be expected to actually do their work.