Y’know, it’s occurred to me that Class is set in a post-World War III post-Doomsday world, so really people should not be surprised at this sort of thing anymore, alien-attack drills should be mandatory in all schools by now
RTD and Moffat both seem to have decided that the cracks erased memory of all that stuff and aliens are secret again.
But post-cracks there’s been the alien cubes and the Cybermen and… probably a few more things I forgot!