Hi B., nice to see another South Floridian on here, in addition to me and Doris Day ..! In regards to your question, I don't think there is a "right" or "wrong" answer here. It's more of a "at your discretion" thing. Follow your gut. Some parents may feel more comfortable about letting their kids attend, while others will say absolutely not. Only you know your child and can follow your own gut. Personally, if it were my child, who is 11, I would say absolutely not. She can get lost, become injured and if she is all alone in the park, she'd probably have a panic attack if she could not find someone in the immediate area.
I have allowed her go to Disney with her cousin and her cousin's friends (high school juniors and seniors, driven there by the aunt), but they all stay together in a group. They all go on the rides together and if one of them doesn't want to go on a ride that the rest do, she stays behind and waits till they come out. I have also done this for my child, as I hate roller coasters and she loves them. I have let her go with another kid and his parents while I waited outside by the exit. She did not want to go alone and I found this was a happy medium -- it gave her a degree of independence, a break from being with mom at all times, while being safe and befriending new kids.
I went to Disney for grad night at night, like your son and his classmates -- day trip, coming back late at night -- but I was also a senior in high school, almost 18 years of age. We also had the park to ourselves after they closed, for a few hours, so it was all just high school kids from our school. Very different! I don't think children at age 12 are equipped to handle themselves and any situation that may come up if they are alone in a large park. I am not talking about rapes or abductions but yes, there could be some creepy person following them around, trying to touch them, or they could sprain their ankle and find themselves unable to walk, and with no one to turn to for assistance, and no way of reporting to a chaperone (most kids at that age, I assume, still do not have cellphones, mine doesn't).
Also, kids that age still get tired quickly, many of them are still on a sleep schedule so by 8 p.m., they are pooped out, especially if they're running around and on their feet all day, and in the heat. I would not see a problem with having a sixth grader attend if there is a chaperone handling a group of say, 8 kids. The chaperone would not have to get on the rides but could wait around the exit and entrance area, and that way, everyone is safe and accounted for at all times. I know my child would feel uncomfortable if she were all alone in the park, with no family or friends in her group. It's not just me being protective, but knowing how my child would react, and I would not want to put her in a situation that scares her. Considering your son will be going to Disney with your family a few weeks prior to this trip, he may be "over it" and may not want to return so soon, so he may just decide to pass on the trip all on his own.