It's fine to pull content from properly licenced sources, imho, too.

I disagree. This would create a tangle of licenses and attribution requirements that will make the contents less usable in/by other projects. We should remove the burden of attribution where possible.

It doesn't have to be a tangle if you set out the rules from the start. Copyleft exists to protect projects and attribution exists to give authors credit for their hard work.

A CC-BY-SA project should be doable. But I'm hoping to examples of situations where educational materials were harmed by choosing an attribution style license.

(also, if it's in a repository, the "who wrote what" becomes a bit easier)

As for pulling in content from outside. Ask. Firstly ask if the author would like their work included, then ask if they could relicense their work to whatever is needed and then finally if the other two were met with enthusiasm, ask them to join the team. :-)

Best Regards, Martin Owens