Sudden School Uniform

In Real Life, if a school is planning a major change of Dress Code, particularly if they're going all the way from "no shirt, no shoes, no service" to a strict uniform policy, they will announce it well in advance, seeking consultation from parents, teachers, the community at large if it's a public school, and often students, with any changes to take effect at the start of the following school year. (This applies to most other major policy changes, too. Schools are run by bureaucracies, and bureaucracy tends to move ponderously).

Not so in fictionland. Maybe the Reasonable Authority Figure has been replaced by someone less reasonable, maybe the principal just snapped, maybe it's coming down from above, but the fictional school now has uniforms. With a day's notice, a week's notice at most. The resolution of the episode will lead to them being abolished since Status Quo Is God, usually reverting to No Dress Code at all.

Often found within a Tyrant Takes the Helm plotline. Primarily seen in American series, since it needs on the one hand a serial medium, such as television, and on the other a cultural backdrop in which school uniforms exist as an option but are the exception rather than the norm. Sub-Trope of Dress Code, and also of Effective Immediately. See also Forced Into Their Sunday Best.

Anime and Manga

 * Ranma ½ had something similar with principal Kuno introducing a mandatory hairstyle (or trying to) in addition to the existing uniforms.

Comic Books

 * Archie Comics had a story arc revolving around this.

Live Action TV

 * iCarly had this when Principal Franklin was kicked out.
 * Degrassi played this straight with the implementation of uniforms over Christmas break without prior discussion, but subverted it by keeping the new dress code in place to date.
 * Little House On the Prairie did this when Mrs. Oleson took over the school. It's possibly the earliest example and memorable because the school board ignored a glaring conflict of interest in the part-owner of the only clothing and dry-goods store in town, forcing parents to buy additional clothing.
 * The series of Ten Things I Hate About You.
 * Shake It Up, "Protest it Up". CeCe lets out a Big No in response, but Gunther and Tinka have great amounts of extra time and energy when they're not planning out over-the-top outfits on a daily basis...

Web Comics

 * This routine happened in El Goonish Shive. Principal Verruckt of Moperville North High School decided to have all of the students wear uniforms (apparently without consulting with parents, the local school board or anyone else) and put the policy into effect almost immediately. It's also a splendid example of Webcomic Time, since the policy was in place for less than a week In-Universe but covered 3½ years' worth of strips (most of which was taken by the birthday party arc).

Western Animation

 * Danny Phantom: Vlad Masters, the newly elected mayor, makes this a rule at the local high school, specifically to annoy Perky Goth Sam, since she's a close friend/sidekick to Vlad's Enemy Mine, Danny.
 * My Gym Partner's a Monkey had the episode "Uniformity", where Charles Darwin Middle School instates a prep-school uniform that causes everyone to act uncharacteristically genteel and proper (read: boring), in stark contrast to the students' usual antics. The uniform was instated in about a week and later revoked in less than a day.
 * The Simpsons. The school uniforms were abandoned when the gray dyes ran to a tie-dye look in the rain.