I would be more worried about my child's behavior than I would be about the bite, personally. Now, an unprovoked attack is one thing, but your son brought this on himself. If it wasn't a bite, it would've been a punch, or shove, etc.
Teachers can't control everything the kids do and biting is COMPLETELY normal for this age. They are still babies & can't express themselves, so some kids bite to communicate frustration or anger, etc. IMO, your son was just as out of control as the other kid & they both need to be taught & reminded right from wrong.
I just saw your other post about the age group, and about the scratches & this is completely normal kid stuff, especially at this age. Kids fall & accidentally hurt each other. It happens all the time. I know it's hard to not nit pick everything, but if you're going have your child in someone else's care and around other kids, you will have to get used to incidents like this.
ETA - so, if your child is 18 mos, then the other one is too, and is still learning, as well. The other child is not perfect & needs to be taught & so does yours. I think some understanding is in order.
I also see where you say your child is "spirited", so it sounds like you have a wild boy on your hands. Please dont' be the parent that thinks their child is perfect & that all the other kids are always at fault.