Are we supposed to think she gets away with it?

And that is precisely how she'll get away with it. I could completely see the therapist being discredited. The Police Officer didn't seem much on her side throughout the movie, so he'd be easy to deceive. She's (Liz) a sociopath and apparently a master manipulator. Those two things put together make her very dangerous, and very easily believable to those who want to believe her, and the cop definitely seemed to want to believe her. All Liz has to do is say that the therapist coerced her to say everything that she did on the tapes, and because the two were found in the hole together by the officer, and Liz immediately plays on the cop's sympathies, she easily can convince him, and the therapist knows it.

Technically she didn't kill anyone. She was negligent because she had the key and didn't let them out. She could be accused of holding them hostage. However, it seems she is a spoiled socialite without a father figure that was obsessed with a boy who lacked interest in anything other than skinny blondes.

At the very least, a lot more people than her therapist would be asking questions about what happened and the families of the other kids would demand an explanation

But she doesn't have to give them one. Or rather, she can give them the story she originally gave, which made it look like she had gone mad and fantasized a happy ending.

Eventually the police will come up with some theory about what happened down there, and she then she can slowly "remember" it.

Short version: she doesn't need a credible convincing story because everyone thinks she has gone round the twist anyway.

I always wondered about other individuals views on Liz. In the first half of the film, Liz paints herself as this loser whose only friend is Martin and who the popular girls use and abuse. But then in the second half Martin describes her as this popular, spoilt, snotty cow who is top of the high school food chain. Surely the investigators would get character references and see whether Martin or Liz was telling the truth? It wouldn't be hard to go to the school and ask other pupils whether Liz was popular or a loner - thus discrediting her

I found the entire movie interesting but implausible. Even if she's a skilled manipulator she can't control Martin while he's out. I'll have to watch it again, but It's not clear to me if he was even involved in the situation beyond making entries in the computer. In any case his story about what he was doing and where he was would check out and stay consistent because presumably it was true and there'd be other people to verify his whereabouts. Liz's story kept changing, and the whole suicide cover was ridiculous. We're supposed to buy that the police instantly came to the conclusion he committed suicide by going over to Liz's house and jumping off a bridge that's two feet above the water? Finally the counselor didn't record the conversation, and she might have gotten in some trouble for taking Liz to the crime scene, but I doubt they would utterly disregard her testimony of what happened over a batsh*t crazy teenager especially if it matched up with what was found at the scene.