My company has an agreement with a local University that every year we'll act as the customer to a team of students and get them to deliver a relatively small, simple software product to us. Our job is basically to tell them what we want, and help them out along the way by clarifying our requirements and answering other questions where appropriate. We set up the sprint schedule and hold meetings with them. We don't manage them as a team or develop alongside them (unless there's an emergency); the most development interaction we have is doing code reviews.
I've been involved for the past two years, and have taken on a more prominent customer role this year.
The issue is that there are some members who are clearly not pulling their weight and are slowing down development, while others are trying hard and are much more engaged. As above, it's not my job to actively manage them (in fact I got explicitly told not to do this); that's done by a student project manager, but soon I will have to give feedback to the team as a whole, which affects their grades. I'm finding it very difficult to give feedback to the team overall because of the aforementioned issue with individual team members. It seems unfair to give an 'average' score to the team when I feel that it doesn't accurately reflect the individuals' contributions; the ones who tried and were engaged will get less than they deserve, and the ones who didn't try will get a free pass.
In this situation, I feel like my hands are tied, and I'm not sure how to best proceed.