Controlling data sharing, visibility, and access permissions within Microsoft Teams for Education
Hi TompsSummper,
Thank you for choosing Microsoft community.
Regarding your concern, please enable External access in the Teams admin center (admin.teams.microsoft.com) for each tenant. I've checked that this feature is enabled by default. You may get more instructions in Use guest access and external access to collaborate with people outside your organization & Manage external access (federation).
Hope it helps! If any update, welcome to share with us.
Thanks & Stay safe,
Qian