I'm guessing you chat with the teacher at least every now and then about how the group is working. I'd mention to her that there's a split in ability level that's starting to show, and that you'd like to offer some more challenging work to the students who were in the upper range. I'd have a few ideas in mind before talking to her, so it's not a situation where you are asking for more work from her.
I'd also suggest trying to find something that's non-worksheet oriented... but that's just me lol!
Best wishes,
polarbear
