which one is correct
the teachers are having lunch in ...
the teacher's room
the teachers' room
the teachers room
If they're having lunch in the room used by all the teachers, you use "the teachers' room".
The apostrophe goes after the letter "s" if the noun is plural.
(If the room belongs to just one teacher, you'd write "the teacher's room" -- but I've never heard of a teacher having a room all to him or herself, so you probably need the plural version.)