Translation:You have to leave your children at school first.
As far as I'm aware (Native speaker, New York City), one would not leave their children to school. "Leave to" is an acceptable construction in the sense that one could "Leave them to their homework" but that means leaving them alone to do it. The only acceptable use of "to" with regards to one's possessions (Yes, children, I know, moving on) is when one is bequeathing something.
In fact måste in Swedish can be both present and past tense. If you want to be unambiguous about the past, you can say Jag var tvungen att …. But otherwise, måste does not change. Jag måste göra det i går = I had to do it yesterday. Jag måste göra det nu. = I have to do it now.
I said leave your children in school which in English would mean the same as at school, unless you actually wanted them outside for some reason. However the sentence seems to have been taken out of context leaving one wondering what comes next which causes some confusion about how to use the word på.