They have difficulties at the school. To be proper I think..
I think 'på skolen' is a set phrase meaning 'at school', not necessarily a specific school.
The English sentence sounds weird. I'd say they either have "a difficulty" or "difficulties".
As for several people having one and the same difficulty - that seems strange, too.
I agree that the plural is a better fit here, and have edited the default translation.
The original is how i hear it and say it, yours are way less common where i live at least.