From Bulbapedia, the community-driven Pokémon encyclopedia.
The Pokémon Ninja School (Japanese: 忍者スクール Ninja School) is an anime-exclusive location in the Kanto region. It is located between Lavender Town and Fuchsia City. Angela is one of the teachers here. There are many students here, including Evian.
Ash and his friends went to the Pokémon Ninja School in order to train themselves in jujutsu. Brock caught a Bonsly here.
It is similar to the Pokémon Jujitsu Academy in Johto.
Pokémon seen in the Pokémon Ninja School