You’re rude.
I’ve been in classes where children used this opportunity to be completely rude to the teacher. I can understand using it in common, every day life situations, but a classroom is large. I’ve also been in plenty of company meetings - no one interrupts the boss with a “bless you” across the room.
I’ve been a Christian for most of my life — I see nothing in the Bible about needing to ward off evil spirits from a sneeze. That’s just hocus pocus. It’s a nice custom, and that’s it.