Hi Javier,
If the teacher account is invited as a collaborator on the whole workspace, it will have access to everything in the workspace.
To restrict access to only some bases and interfaces, you could first remove the teacher account from the workspace-level permissions, and then invite it as a collaborator directly for the individual bases and interfaces you want the teachers to access. That way the admin account will still see all the bases in the workspace, but teachers will only see the ones they need.
Does that make sense? As long as it is the same single teacher account, it should only be billed once for having access to any number of bases in the workspace.
Hope that helps!
Rowan