Which one is correct? I think both are grammatically correct but some teachers say, "after "when" we can't use "will".
- I don't know when the meeting (starts - will start).
Yes, both are correct. "Starts" is preferred for scheduled actions (Cf The train leaves at 7), and "will start" is just future.
Those teachers you mention might be confused with the case where "when" introduces an adverbial clause of time, where "will" cannot indeed be used, even if reference to the future is made. In the case at issue, however, "where" introduces an indirect question. Compare:
- When will the meeting start? -> I don't know when the meeting will start. (Indirect or reported question)
- I will be ready when the meeting starts (when the meeting will start). (Adverbial clause of time)