I have an assignment that is 30 percent of my grade. Would my teacher say

And why?

[EDIT includes minor corrections]

• Possible duplicate of Have a seat, guys (have seats?) Aug 21, 2016 at 15:05
• In this case, I think grade sounds better, because each student has only one grade. Aug 21, 2016 at 15:07
• @Alan Carmack: I disagree. Because OP's text explicitly pluralises the pronoun (by attaching the Saxon genitive to you guys rather than using the ambiguously singular/plural your), only grades really works. Aug 21, 2016 at 15:25
• I can't imagine a teacher using you guys'...
– J.R.
Aug 21, 2016 at 18:43
• @JimR - Just to clarify, I meant that I can't imagine a teacher using you guys' in the context of explaining the grading scheme while going over the course syllabus on the first day of class. (Something like, "You guys don't forget that Project 2 is due on Monday" while the students are filing out the door on Friday afternoon wouldn't seem unusual at all.)
– J.R.
Aug 22, 2016 at 15:42

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.

• So in the "grade" case, they think of the class as one entity like referring their grades to one grade, which is like a grade composition template.
– HUN
Aug 22, 2016 at 2:51
• I think that you are saying the same thing, yes. Or something similar. Aug 22, 2016 at 9:12

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...