Boy Demon Lord Warns a Bully
Leonis is a defeated demon lord whose 1000 year reincarnation spell malfunctions on activation and resurrects him as a 10 yr old. He gets “rescued” and ends up enrolling in a magic academy while he prepares to reconquer the world (at least that’s the plan lol). Anyway, in the clip he warns off a bully messing with one of his new companions.
The anime is “Seiken Gakuin no Makentsukai” (The Demon Sword Master of Excalibur Academy) and the clip is from episode two.