Ninja School

Yeah, my graduation party was crazy too!

"A secret ninja school? Wait, is it a public secret ninja school?"

—Ron Stoppable, Kim Possible

Learning how to be a ninja is rather straightforward. Normally one learns from a personal master, or sometimes a dojo. But there are those that believe that learning the skills of a ninja can be made to look like a public school. After all, you call your ninja instructor "sensei", which means "teacher", so it's the same thing. Right?

Calling the instructor "teacher" is about the only thing ninja training has in common with schooling. There are no written tests, homework, or grades. You learn the skills and move on when you master them. Don't expect graduates of this Ninja School to act like real ninjas.

The comic Ninja High School is the Trope Namer, although it is not (usually) an example of this trope.

Examples of Ninja School include:

Anime and Manga

Naruto has a three-year Ninja School in Konoha, and other ninja schools are assumed to exist in the other Hidden Villages (although little is known beyond that supposition).
- We know Hidden Mist has one because Zabuza graduated early before he was even enrolled by killing off an entire graduating class of students.
Nintama Rantaro is a rare example of a Ninja School that actually teaches actual ninjutsu techniques and has students, graduates and teachers that act like real ninjas (when they carry out their missions). It's reasonably realistic.

Film

Ninja Academy is a spoof of this trope.

Literature

Discworld: The Assassins' Guild in Ankh-Morpork would probably fit the trope as well, especially considering the inspired final test in Pyramids. On the other hand, the young gentlemen (and recently ladies) who study there receive such a well-rounded education, many upper-class families enrol their children only to withdraw just before the distasteful "final exam".
To Be a Ninja is a series of novels by Benedict Jacka. A brother and a sister escape a drug baron father in a secret ninja school in Britain country. A third of the pupils are British, the others Japanese.
They are never called ninjas, but the House of Black and White in A Song of Ice and Fire is basically a ninja school where acolytes are trained to become assassins.

Live-Action TV

In one of the Ninja Specials of MythBusters, Grant, Tory, and Kari try ninja weapons and fail. The narrator suggests that they won't be passing Ninja School anytime soon.
Power Rangers Ninja Storm has at least two, the Wind and Thunder Ninja Academies. Both are attacked in the season premiere, with only a handful of students avoiding capture - the rest spend the entire series held prisoner on the Big Bad's spaceship. Despite the elemental names, the Wind Ninja Academy teaches manipulation of multiple elements—Air, Earth, and Water. The Big Bad talks about the ninja schools as if there are way more than the two, or were before he wiped them out. The Super Sentai twin, Ninpuu Sentai Hurricaneger, is similar.
- That said, it's never clear whether the academies are full-time schools or just afterschool classes - it's impossible to tell since the regular lessons were suspended while the entire student body was missing. The Rangers did mention homework from time to time, but never said whether it was ninja homework or regular homework. We do, however, see an academy graduation, which is similar to normal school right down to getting "diplomas".

Music

The Scottish indie pop band Bis did a song called Ninja Hi Skool

It's cool! It's cool! It's cool cool cool! It's cool cool cool for YOU! NINJA HI SKOOL!

Web Comics

Therkla of Order of the Stick mentions that she was valedictorian at Ninja School. A flashback panel reveals that she took the title using her graduation cap.

Therkla: I am salutatorian -- no more!

- The Order's waiter at "T.G.I. Wednesday's" in Azure City mention she's working there to put herself through ninja school.

Web Original

The Yama Dojo in the Whateley Universe, which has also trained some Japanese mutant children in ninja techniques. Some of them didn't exactly learn everything you'd hope.
In Ninja Academy: WTF Journal Prequel there is a Ninja School
Ariadnio in Greek Ninja is this.

Western Animation

The Nickelodeon cartoon Shuriken School.
Ron Stoppable in Kim Possible gets a special episode where he enters an exchange student program and gets sent to one of these. It partially averts this trope by being a live-in dojo secluded in the mountains, but it still has classes and school staff.
The Huntsclan Academy in American Dragon: Jake Long. Rose even showed Jake that, despite the fact the students there are trained to slay magical creatures (there's even a class named "Dragon Slaying 101"), it was still a school by starting a rumor (Jake needed a distraction).