I hate group assignments for this very reason. I know Helen has had them and ended up doing the bulk of the work because the others can't ever get together to get it done. If that was the only time they could rehearse, then I think you did the right thing, but like Rachel, I might have asked the teacher to set aside some time during her class, first, for them to rehearse. After all, it was her assignment causing the problem.

Good luck.
L.