@RBourgeois -
What @RobDitto originally posted is correct and works very well for what the original poster wanted. You make an assignment and assign it to 20 people in the class only. The other 980 will not see the assignment and will not receive a grade.
In your case, I am not sure how the above would work with power school (I don't use it or know how it interacts with Canvas). As to your question about the easy way to assign a grade or scoring criteria to all students, there is a 50% solution - you can set a default grade for all students. The reason I said 50% is that it is only numeric grades as far as I know, i.e. you can't set the default to ex for excused.
I am not completely clear about your task where you state that you have a default grade for assignments, but then you state that you don't want it sent to PowerSchool.
However, with a little bit of work (not the easy solution you and everyone else would like to see) I believe you can do what you want. Here I will assume that you need to assign excused to all the default grades you don't want sent to powerschool(once again I don't know how it interacts with Canvas, so the following may not work at all).
1) set the default grade for the assignment(I am assuming this default grade is something that is not counted)
2) export the gradebook and in the file delete all assignments but the one you want to modify
3) do a search and replace on the default grade and replace the grade with ex (for excused)
4) save the changed csv file and then import it back to the course. All students with the default grade have now had it changed to excused.
Let me know if that is what you are asking. Or, as I suspect, I am completely off base.
Ron
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