This one manga on a website that i don’t remember the name of was about a boy summoning a monster as a companion and ends up summoning a demon as one. She (the demon) becomes his companion after telling her story about how demons can’t have human companions because of their power. They end up getting to school and defeating other people with their monsters and disguising so that the demon won’t be called out. (This one is a maybe but I think it was an ecchi)
Vermeil in Gold - A Magician Pushes Through the Magical World With the Strongest Disaster (warning, TV Tropes entry):
At a Wizarding School, Alto Goldfield must summon a Familiar or else be held back a grade. Having failed to summon one each try, he finds an old grimoire and tries the summoning spell inside. He ends up summoning a powerful, seductive devil named Vermeil. He asks her to pretend to be human to not scare anybody, as she can conceal her horns and tail, but this causes them to come under scrutiny as humans have never been summoned as familiars before. The two must get through the school year amid several people picking fights and Alto's jealous childhood friend, Lillian Kudelfelt.