well, based on the fact that I wasn't bitten, I'd say it was a nurse shark. I'm basing this on a separate incident that occurred about an hour later.
I had been wading around in neck-deep, very murky water, which is flat out nerve wracking anyway. After the bump, I went into the beach and grabbed a board to finish the lesson. On the board is just way safer, and in deep water it's easier to get around on a board. So, with about 10 minutes left in the lesson, I got this kid into a really good wave.
After his wave, I submerged the tail of my board deeper into the water to spin around 180 degrees and paddle back out. (When you push the tail of your board down in the water (while sitting on it), when you turn around, the buoyancy of the board helps propel you forward). When the tail of my board submerged, it hit a substantial object, that was very clearly alive. I could feel a ton of turbulence from the animal as it moved to get out from under me.
If it had been any other kind of shark, it would have bitten me at the point my board hit it, IMO.