Nitrogen Oxide - Advice needed urgently

So I'm actually a Maths teacher, but I do 7th grade science. The other teacher told me to do this experiment with putting nitric acid on a coin. He made it seem like it would just bubble and change colour. Little did I know it would give off loads off nitrogen oxide brown vapour. It just spread out across the table, staying at that height. I got the kids to stand back, and I don't think any of them breathed in loads directly. However, I've been scanning the internet for everything I can find about it, and it seems like it could be really harmful! I'm kind of freaking out, especially the descriptions about it going unnoticed for a while, because no one seems to have anything wrong with them at all right now, yet..

I asked the other teacher, and he said its probably fine, but can someone who actually knows about this stuff tell me how serious this is?? Should I just calm down? Or do I need to urgent get all of them to hospital??