5"How was your day at school?"
This is fine in BrE.
"How was your day in school?"
This is possible in BrE if asked of a person, for example a governor or trainee teacher, who had spent one day in a school visiting or observing.
And I am in class now. VS. I am at school now. Let's meet after class. VS. Let's meet after school.
Only the two in red are common in BrE. We can also say, "Let's meet after the lesson".