1. If your preschool gave you a book of guidelines check it out. Sometimes they are fairly specific (don-t wear makeup, no nail polish, etc). If they say things like this you may want to wash off the tattoo
2. As other have said, what-s the tattoo like? Is it a flower? A dancing bear? An American flag. Those are all probably OK. Now, if its a "Vote no on Prop 8", Gun, Half naked women etc. Probably not
3. Would other students have it? If there was a birthday party or something similar and many students got them then it is probably fine.
4. This one is a bit random, but is the tattoo at all gluten based? Some stickers and stamps use gluten in the coloring process. Some of these are banned from schools so that a gluten free child won't lick your sons hand and get sick. Now I think the odds of this are very low (and I think children should not be licking one another) but something to consider
I wouldn't worry too much about it. If the school is adamantly against it they can remove it for you and let you know at pickup.
Too much time spent at preschools (I taught)