J.C.
I am a K teacher and I had a extremely bright student who did the same thing. It was a control thing with him. He only did it at post K daycare. I think the best thing to do would be to...
1. act as if it doesn't bother you (as hard as that may be) it is very possible he is doing it to get some extra attention from you.
2. make it as inconvenient as you can for him (when my daughter has a wetting accident I make her take her clothes to the laundry, wipe up the floor, take a bath, etc.)
3. later have a talk about it (tell him that you want to help him solve this problem and why)
good luck!!
I highly doubt he would do it at school! don't worry about that + we K teachers deal with all kinds of things discreetly...we are used to it!