Motivations of the teens, do they make sense?
From what I gather, most people conduct their lives based on what they feel at the time, rather than a set of guiding principles. It seems like a lot of people who watch movies are expecting the characters to react how they themselves would. If this were the case, it would make for very boring cinema, as all characters would essentially be identical in behavior.
People have no idea how they would react either way, because they don't relate with, or in many cases even empathize with, the emotions governing each character's behavior. They assume the scenario is playing out in the movie the same as it is for the outside observer. That simply isn't the case. Real experiences feel different than just watching a movie when you were bored.
Among Ricky's guiding emotions are an unhealthy fixation on Joanne, sympathy for Deadgirl, and a fear of getting arrested. He wants to do the right thing, but won't take personal risk to back up his convictions. If he had, she would most likely have been "rescued" only to be further experimented on in the name of science. An outcome many would accept, who believe ends justify means. However, it brings into question the morality that says it's okay to torment her in the name of science, but not for other personal gain.
It would be an entirely different movie, possibly where some perverted scientists do the unimaginable things, and she breaks out of a laboratory or something. A sci-fi version much less gritty and "real". Who knows, maybe it would've been a smashing success.
Now on to JT. People actually go about this character as if he's supposed to behave normally. He's a pervert and a sociopath. He probably sees all girls as unfeeling zombies, because they don't feel anything TOWARDS HIM, so from his perspective they are unfeeling. This lends culpability to girls like Joanne, the attractive and popular girls who feel shame when associating with "low status" people.
Joanne, while the most sympathetic character, is intended to have flaws. The pursuit of popularity is actually a manifestation of low self-esteem and a need to be validated. Even though she is extremely beautiful, she is insecure because every morning she wakes up and puts her makeup on knowing her whole life is all an act. I think she was interested in JT at first, and wanted to win him over, but then realized the "game" was over because all his cards were on the table, and she won. Women respond to a challenge. He also put her on the spot and sounded approval-seeking when he offered to take her out sometime. This killed any previous attraction. An example of emotion governing behavior.
The jocks, are similar to Joanne in that they are concerned about their self image. However, the pressures of society on men are different to that of women. They are susceptible to challenges on their masculinity, even feeling the need to "prove themselves" to beta males. They were told Deadgirl is just a very freaky girl who likes to be tied up and gang raped. One jock is into it, the other is reluctant but caves to pressure from his friend.
Wheeler is just as susceptible to peer pressure, just not a jock and definitely not one of the smart kids. He just wants to fit in somewhere, anywhere, and JT presented an opportunity.
Though execution of these ideas might have been better, the characters are a demonstration of many ranges of youth psyches. None of them are intended to be "functional" psyches, which honestly would be asking a lot of today's teens to say the least. The point this thread is intended to make, is that these characters have clear motivations and rules for how they behave.