Can you answer these questions?

They are all correct. He is at the University = Means that he is at that location.

                                    He is at University = Means he is a student who studies at university level.

The same applies to the girl (she) She is at the school = In the building (location)
                                                            She is at school = Because she is a student ( for a period of time ). Later she may become a student with the University.

how do you call a school teacher? teacher or mister or both

In the UK a teacher is usually 'Addressed' (called) Sir, Miss or Mrs. However, more frequently and less formally these days. Their title Mr, Miss, Mrs, followed by their surname e.g. Mr Smith. Miss White. Mrs Jones.


Thanks.

Sorry: I should explain Mister is usually written as 'Mr' but spoken as 'Mister'.

Mrs is written this way but spoken as 'Missus'

Can you answer these questions?

In the US 
he is at the university, but not he is at university

but he is at/in college would work as a general status, a particular college would be at the college

same for school.

in the US we would not say "Teacher Jones" we would say Miss Jones or Mister Jones.