Alright, but bear with me.
Like, they haunt the school because it was built over the old Musain (ok, ok, I know it actually is like, in the Alps, but hey), and ended up coming back “to life”, so to say, because of an unlucky student’s potion that ended up being thrown out the window and fell directly on the ground, so suddenly there’s this odd bunch of revolutionaries wandering around the castle, being completly mistified (and excited) about all the magic stuff.
Combeferre usually haunting the library trying to get all the knowledge of magic he was never able to have.
Courfeyrac charming the first-year students while helping them out to get used to the school, sometimes randomly sitting at someone’s side during meal’s time if they’re feeling lonely.
Joly always appearing when it’s raining and someone’s trying to go out without an umbrella, ready to give a lecture about how they’re going to get a cold and you really should go back inside this instant young one.
Jehan helping the gardeners with the flowers, and always ready to weave some in someone’s hair while murmuring/discussing poetry to/with them.
Bossuet finally not tumbling over everything because now he’s incorporeal but even then managing to knock things out of it’s places accidentally.
Bahorel ‘tsk’-ing at the school’s robes and leaving affronted letters about it in the principal’s office. Or challenging first-year students who don’t yet know he’s a ghost to a arm-wrestling match only to laugh madly when their hands don’t encounter any resistance because he’s a freaking ghost.
Feully sitting in the back during classes, or doing origamis and tricolour cockades that the students collect and/or enchant.
Grantaire interrupting said classes with long speeches about nothing, to the ire of the teachers, or being the one who knows every nook and cranny of the school-grounds and the small magical village close-by, always managing to just be there when a student gets lost or is trying to sneak out (which he usually helps them to, if they bring some “offerings of wine to his poor, tormented soul, left to wander these empty corridors without ever finding rest”).
Enjolras having passionate discussions with the professors and students about the estabilishment and how the british magical system of schooling is prejudicial, why should someone segregate people in ‘houses’ it’s just wrong.
Les Amis as Beauxbatons’s ghosts