I've managed to get ours working - hopefully this will fix it for your schools?
I made a change to the messaging policy that I had assigned to the students - (Education_SecondaryStudent) then I re-applied the Policy Package (Education (Secondary school student)) to all the students. Teachers were then able to activate their Teams.
Even though all the students in an affected Team were displaying having the correct policy package/messaging policy assigned, i don't think they had been assigned correctly at the time - when there were provisioning issues at the start of remote working