My dog is acting aggressive

You definitely should be contacting the seller about this, regardless. Whether you can give the dog back is largely dependent on your contract with the seller, and what guarantees (if any) the seller made regarding temperament. That said, and if I had to guess, your seller did very little or no socialization with this dog. That should have been question 1 when interviewing the seller. If you don't have a contract, the behavior can be modified, but a certified professional would need to assess whether this dog under your guardianship is a candidate for behavior mod.

I agree with Curbside (but don't tell him) I would definitely let the seller know of this issue. That said, more often then not a puppy can improve it's social skills. Bringing a puppy to a large group of children is pretty overwhelming. I don't know how kids can be. Loud and they move herky jerky. Even if just sitting in a circle around the puppy could certainly have scared him, he may have felt trapped. How is he with your son at home? Have you done any other social outings since you had him?
I got a Doberman puppy from a drug house that was around 10-12 weeks old and she growled and snapped at me at first too. She managed to become a very social dog. I took it slow and set up many successes for her to build her confidence as her aggression was coming from fear. Good Luck with this puppy. Let us know how things are going.

It sounds to me like he's scared. I'd say he wasn't socialized properly.

He is really good at home. He loves my son and when someone comes in after he acts crazy with the growling and barking he's fine. I just really wanted a dog that I could take anywhere. I love him, he's really sweet and I guesss I should have rethought the whole taking him to school thing but I thought he's a puppy, puppy's love everybody. I did send the seller an email about it but she hasn't gotten back with me. The people that sold her to us are really great people, they don't breed all the time they just have litters here and there not something they do all the time. I'm going to try to work with him, I called a dog training place here in Memphis and they said they could correct this behavior for us but it's pretty expensive so we are going to see how it goes.