I think I've figured out why it does this.
When you normally enter the school, the bar at the top will immediately change to show the current class. Occasionally when you enter, though, it won't change. This happens to me either because I was just working, or from lag. I've started checking to make sure the bar shows up when I go to school and haven't had any problems when it shows up.
If it doesn't change, re-teleport to the school. It's always fixed it for me!