Hide Group members' work from other members

I am using 2.8. I really enjoy the group facility. I have 105 grade 8 students, who are in 5 different classes (A, B, C, D, E). I put the different students into into separate groups to facilitate grading etc.

But, because they are in the same group, they can see each others' grades. How do I prevent this - ie make it so that a student can only see his/her own work/grades etc, while still being a member of a group?

Thanks,

Peter