YMMV, but isn't it the complete opposite?

Open source projects do not, and have never, gone out of their way to teach these things.

Ofc doing it for a grade is silly

But if you contribute to open source, you must follow the etiquette or your PR is denied etc.

What inexperienced programmers get from open source is that experience where they better learn how to fit in effectively, or they get bounced.

In the nicest possible way, open source is a great way for people on the HS/college track to take responsibility for themselves.

It doesn't require much active effort on the part of the open source community to deny a PR or exclude a bad apple if that makes sense.

And I'd say the majority of open source communities long ago put systems in place to deal with bad apples (or should have).