We say either caretaker or groundskeeper (but not just keeper by itself) in the US.

In the UK, the school "caretaker" takes care of the school and does things like open and lock it and carry out repairs. Another word for "caretaker" is "janitor". "Keeper" is not used instead of caretaker" in the UK and I have not heard it used in this way. It would be useful to know if "keeper" has this meaning in other English-speaking countries.