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.
Born on the same day: "It's our birthday today."
Born on different days: "We should meet on our birthdays."
The reason is that, in the first case, there is just one day. Therefore, birthday should be singular. In the second case, it should be plural because there are two days (say, June 3 and October 7). Hence, birthdays.
Best Answer
Both are correct in their own senses.
And as for your question about the project being your own, the subject that we speak of in the sentence is not the project but the teacher, and as such your class teacher is the teacher of all her students, not just you.
If the subject were a project, like:
Here, we would use my instead of our because it is solely your project, not that of the class or a group of students.