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".
Student or Learner