"Separate students according to their academic performance(s)."
(For example, their test results.) Should I use singular or plural in this phrase?
I did some research, but the example here is quite different than mine.
In your example, the singular "performance" could be used
Separate students according to their academic performance (into different groups).
according to each student's individual academic performance
which would result in
The academic performances of the students in a given group will be similar.