1. You are teaching a low level of students?

2. You are teaching low-level students?

3. You are teaching lower level students. (<=> higher level students.) (cf. middle level students?)

I think #1 and #3 are correct, aren't they? and its' opposite and cf are also correct?
#2 and #3 are OK. #1 is not.
3. I have experiences teaching different level students.

4. I have experiences teaching different grade students.

Q1) Are they correct to say?

Q2) Is it correct to say 'I have experiences ~ing'?
Students: We have free audio pronunciation exercises.
3. I have experience teaching different level students.

4. I have experience teaching different grade students.

Q1) Are they correct to say?-- They are awkward formations but probably heard.

Q2) Is it correct to say 'I have experience ~ing'?- Yes, as corrected.
Regarding questions No.1, is "...students who are different level/grade" more natural to say?

Or.. what about this? "I have experience teaching students whose level/grade are various?
Regarding questions No.1, is "...students who are different levels/grades" more natural to say?-- Yes, that sounds easier on the tongue.

Or.. what about this? "I have experience teaching students whose level/grade are various?-- No, this doesn't sound natural to me.

Overall, this is not an easy structure to evaluate. Trying to come up with what I say results in this:

I have experience teaching students of different levels.
Teachers: We supply a list of EFL job vacancies
Mister Micawber I have experience teaching students of different levels.
Before I wrote the previous sentence, my american friend said "I have experience teaching different grades of students"

Is it also correct?
Look, moon—in conversation, a speaker could manage to utter just about all of the permutations that we have mentioned in this thread. I have tried to give you the least awkward ones, so please try to use those.