Hi I am not sure how they have done it, but when they go to the link to create a free Teams account, they arent able to use their school email address. Somehow, they have managed to create an account using I presume a consumer email address, add members (other students) to the Team, and they can sign into Teams using this newly created account and this because its not in our O365 environment, goes completely unchecked. A big safeguarding issue.
Yeah! They have probably used their consumer addresses! This is outside your scope of control! Also, nothing you could be held responsible for! They could just as well create a slack team with school friends , use discord or any other app/chat!
yeah, since they aren't in you tenant there's not much to be done, technically if you don't convince Microsoft to remove the teams free edition :) As I said, the could start a whatsapp group and use in school with whatever account outside of their school account! Network wise you can control stuff from your corporate network but that don't apply on this scenario
Content wise there's security and governance features in Microsoft 365 gaining certain control and security of your school content, but I don't think this really apply to your issue