I have an assignment that is 30 percent of my grade. Would my teacher say
This task counts toward 30 percent of you guys' grades or you guys' grade?
[EDIT includes minor corrections]
We can and do say both of them in standard English.
My sense is that most or many teachers who say that would probably not think about their choice of using grade or grades, and most students who hear it would probably not notice which choice the teacher made.
This is because we can think of the grade (or grades) and the situation in three ways.
The first way is to think that every student will get a grade, so there are multiple grades (every student will get one). Since the noun names more than one grade, we can use grades for the same simple reason we say Here are some apples.
The second way is to think of the grade is as a singular thing. It is a thing that the teacher calculates in a certain way, and the teacher is explaining to all of you how she calculates it. (That singular thing.)
I will illustrate this with an example. Suppose a fire department hires 15 new firefighters. On their first day, a trainer might greet the group by saying Welcome, everybody. Let me start by telling you some things about the (or your) job.
Here, the trainer is talking about (thinking about) a single job: firefighter, even though there are 15 people who have 15 new jobs.
Finally, we can conceptualize the message as the teacher speaking to each of you individually, about your grade, and only your grade, even though she is speaking to each of you all together at the same time.
Because OP's example usage explicitly pluralises the pronoun (by attaching the Saxon genitive to you guys), only plural grades really works. (Note that you guys is a very informal usage itself.)
To illustrate that principle, compare...
1: 80% of your grades are based on course work projects
2: 80% of your grade is based on course work projects
3: 80% of the children's grades are based on course work projects
4: ? 80% of the children's grade is based on course work projects
If it had used the ambiguously singular/plural your, it's simply a matter of style / emphasis on the part of the teacher. Does he himself think (or does he want to encourage the students to think) that the class is a collective coherent entity capable of being "idealized" and distilled into a hypothetical single representative student, for example?