What do you call grinds in Chinese?
If grinds mean tutoring/ tuition in this context, we can just simply translate grinds as 补习.
If you need to specific, you can say 校外补习  (outside school tutoring) or 私人补习  (private tutoring)

I give maths grinds to 3 students. - 我给3名学生做数学补习。


I offered free grinds to my students. - 我向学生提供免费补习。


Giving grinds is good money. - 给人补习的收入不错

Usually, 给人补习 (give tuition) is a job for 家庭教师 (private teacher), but anyone qualified can offer tuition to anyone who needs it.