Been there, done that, ant have the t-shirt to prove it. Only difference is that I was in the pattern with my student and one of the other instructors noticed that fuel was spraying out of the left right wing. The funny thing about it was that I was just making fun of another instructor that had the same thing happen to him a couple of weeks prior.
We used to have to taxi to the other side of the airport to refuel at the gas pump. Typically after a student had soloed, we began sending them by themselves to refuel. When these incidents occurred it usually was because the student left the gas cap sitting on top of the pump.
A few smacks to the back of my students head prevented him from ever doing that again.
