There is no School Day, in general. A school day just means a day at school, a working day, in that sense. Hope this helps! :)

It means exactly that, school day

A school day is just a day when schools are open, but sometimes adults also use it - or school night - to express a working day (Monday to Friday). For example "I can't stay at the pub too late tonight, it's a school night". What's the context of "School Day"? Not sure about this one.