Question

1. Where is the restroom?

2. Where is the grils' room?
3. Where is the boys' room?
[Can we use those expressions at school? What other expressions do you have? Does 'the restroom' mean that there is only one restroom?]

Answer 1

All are fine if you correct the spelling error in 2 -- girls'

Using "the" in this common phrasing doesn't mean there's only one. In a school, there are probably many.