Translation:You have to leave your children at school first.
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.
Unless Swedish has changed considerably in the past 5 years, you would not use måste in the past tense. You would use behövde.
Jag behövde göra det igår.
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.
To leave your children in school usually means that you're not withdrawing them from school. Leaving your children at school means that you're dropping them off. The Swedish sentence means that you're dropping them off.