Requisite Magic

Level: Advanced Creature Magic
Subject: CoMC & Hogwarts Magic

House-elves have their own brand of magic that only they have access to, as do many other magical creatures. The upper limits of Elf magic are not known, however, it is incredibly versatile and allows them to perform very powerful magic. They are often underestimated however because of their small stature. Their loyalty also prevents them from using magic against their masters until they are free. ~ Wikia

Game Canon
+ House elves are not free at Hogwarts
+ They are rarely seen by students & staff
+ They clean up common rooms, classrooms, and corridors magically
+ They are kept in the basement kitchens out of sight.
+ There are about 50 house elves at Hogwarts total.
+ They have names like Tippet, Lilypad, Podswoddle, and more.