From Bulbapedia, the community-driven Pokémon encyclopedia.
The Pokémon Jujitsu Academy (Japanese: 忍法学園 Ninja Arts School) is an anime-exclusive location found in between Goldenrod City and Ecruteak City. It appeared in Ariados, Amigos. Many young Trainers and their Pokémon come to the academy to study the principles of Pokémon Jujitsu. Students learn about moves such as Spider Web, Stun Spore, Double Team and Spikes, all moves in the "Pokémon Jujitsu Tradition".
When Misty took a walk with Togepi through a forest, she saw a Pokémon hiding in the bushes. Scared, she called Ash and Brock, who found out it was an Ariados. Then Aya, who had previously appeared in The Ninja Poké-Showdown, showed up. The Ariados was revealed to be owned by Tōkichi, the owner of the Pokémon Jujitsu Academy.
Ash and his friends followed Tōkichi and Aya to the Academy, where Tōkichi told them all about it. Ash, Misty and Brock were allowed by Tōkichi to take classes at the academy for one day without having to pay anything.
The uniforms worn by students
The uniforms worn by the students indicate what classes they take. Students in red uniforms study about battle techniques, students in yellow uniforms take tactical classes where they try to make moves more effective, and students in blue uniforms study about health and nutrition. This last study is very special however, as it is given only once a year and people from all over the world come to the academy just to take these classes. Ash took the battle classes, Brock the tactical classes and Misty the health and nutrition classes.
Much to her dismay, Misty found out her classes involved Spinarak using String Shot to make beauty masks. Brock, in his classes, took a test of popping balloons by use of his Pineco's Spikes. In the battle classes, Ash battled with his Bulbasaur against Aya's Venonat and proved victorious.
That night, all students were robbed from their Pokémon. It was quickly revealed that Team Rocket was behind this. With help of his Ariados, Tōkichi managed to take the stolen Pokémon back. Aya proved her strength and helped defeat Team Rocket.
It is similar to the Pokémon Ninja School in Kanto.