I can't believe they're doing this! You may copy each others' mannerisms more, but having experience with friendship is a hundred times more useful than just being around NT kids who tend to be abusive and are not currently your friends at all.
If you read people's posts around here, you'll see that the greatest distress associated with AS is the problem of social anxiety and abuse--both of which occur much less in a group of people who also have neurological oddities.
Associating with other autistics helps me a great deal. We swap ideas about how to deal with the world; we hang out without worrying too much that we'll say or do the wrong thing, because everybody else is socially awkward too. If somebody else has the same trait that they've learned to work with, they can teach you how to do it too.
Until I first met other autistic people, I had no idea how to solve some of the problems I was facing. Professionals can only know so much about ASDs without ever having experienced it themselves. If you want theory, go to a professional; if you want practical advice, go to another autistic person.
If your parents want you to have experience interacting with other people, there's no better way to get that experience than with other people who also have ASDs. Okay, so you might learn a new stim; but if you also learn how not to have a meltdown when the sun gets in your eyes, or how to handle the problem of an intrusive aide, or how to interpret some weird NT action that you're puzzled on and somebody else has figured out already (and it wouldn't occur to an NT that it was anything other than obvious)... then it's definitely a net positive.
Also: Who's more likely to be emotionally stable and accepted by people:
Someone who's constantly afraid that he will make the wrong move, and so shy that his fear keeps him from interacting?
Someone who's been hurt so often that he's turned around and become hostile in preemptive self-defense?
Or someone who is obviously odd, but comfortable with who he is, and willing to reach out to others?
Give you a hint: It's the third one. People find it much easier to accept someone who is odd and makes no bones about it than someone who may either cry or hit you if you trigger the wrong bad memory.
Get your parents on here. Let us at 'em. I've got about a million reasons why it's not harmful to be with other autistic people, and my own sanity and increased confidence is one of them.