I've read over a few of the stories for the High School AU.

An AU must change canon.

One of the definitions I see people use for the term AU or alternate universe is "didn't happen in canon", but this honestly is a misconception. While it is true the events in an AU "didn't happen in canon" not all stories with events which didn't happen in canon are actually an AU. The first rule of thumb for writing an AU is the story must change some element of canon. The story is thus not an AU if canon isn't changed.

Prequels and sequels to canon material as well as stories which fill in gaps in the canon story line "didn't happen in canon", but don't constitute an AU if canon is not changed in some manner. These works can become AU if more canon material is released that changes canon, but it isn't an actual AU until there is something in the story which is different from canon. Despite this I've seen a few of these stories submitted for the AU contest when it was held.

I'm also seeing a lot of these stories for the High School AU/College AU contest as well. The point isn't to write a sequel to the canon storyline. Let's say for example the characters are in Middle School for the canon plot line, so the writers are writing a story about when the characters go to High School. The problem is the canon events happened, and thus nothing in canon's been changed. Thus the story can't count as an AU as it is a sequel to said story.

What is a High School?

Another mistake I've seen people make is to assume that high school is comprised of grades nine to twelve, and the students range in age from fourteen to eighteen. This is because the high school everyone typically thinks of is because the US high school which typically is comprised of grades nine to twelve, and has students typically ranging from fourteen to eighteen. Some high schools in the US though go from grades ten to twelve, and have students typically ranging from fifteen to eighteen.

The word High School is simply another word for a school which provides a student with a secondary education. Primary education comes before, and tertiary comes after. A College AU thus would be any school which provides any kind of tertiary education for their students. Universities are only one example of the possible school scenario for tertiary education. Other options includes trade school, and service academies.

The age range for the students is going to vary from place to place, as is the education stages used. The US system uses grade levels, while the UK uses years. On top of this high school in the US is usually the ninth or tenth grade to the twelfth grade, while in the UK the high schools run from year one to seven which would be equivalent in the US to the grades sixth to twelfth. In Japan High schools are typically grade ten to twelve equivalently. Italy's system on the other hand is rather complex.

Another assumption people make about high schools is that the school needs to teach a standard curriculum consisting of English, Science, Social Studies, Mathematics, Physical Education, Foreign Language and Elective courses. Not all high schools have this kind of course load. Private schools with religious ties are known to have Theology built into the curriculum. Agricultural schools focus on agricultural learning. Physical Education is cut out as the students instead learn about growing healthy foods, and they exercise by actually putting what they learn into practice.

Setting is what changes.

The third problem is some people believe a High School AU simply needs to be an AU which takes place in a high school, and a College AU simply needs to be an AU which takes place in a college. Shows like Beverly Hills, 90210 already takes place in a high school, so doesn't this mean one simply needs to have an AU plot for the story to be considered a High School AU? Thus the definition of a High School AU can't be simply an AU taking place in a High School setting.

Let's say however you have a fandom which takes place in a high school setting with some kind of supernatural element. Does stripping the original canon of these supernatural elements suddenly make the story an AU? The answer is yes, doing so does make the story an AU, but this doesn't make the story a High School AU or a College AU. This type of AU is actually called a No Magic AU, or a All Human AU depending on what is removed.

The point of the High School AU is to change the canon setting for all of the characters, and the same goes for the College AU. A few may argue that changing a non-traditional high school setting to a regular high school setting counts as it's a change of setting, or changing the high school setting of one country to one of another country. This though is not so.

First – the guidelines for this contest clearly state a "High School AU is a fanfiction story that is set in the alternate universe of high school." It doesn't say "regular high school", or "foreign high school". Second – as I pointed out the definition of high school simply means a place where someone gets their secondary education, and there is no standard to how a high school should be. This so called regular high school is a stereotype.

Facts...

– Not all high schools call their education stages "grades". The term "year" is actually far more common, and another word used in some countries is "class".
– Not all high schools have an age range of fourteen to eighteen. Standards for age range actually vary greatly from country to country, so there is no "regular" standard. Add to this the fact there is no standard for the US for which this so called regular high school is supposed to be based on.
– Curriculum varies from country to country, and in the US there is no national standard at this point. Schools with non-traditional curriculum are still run the same way schools with traditional curriculum are run. The only real difference ends up being whether the school is a boarding school or not, and that's not really much.

Thus it is not possible to write a High School AU for series like Harry Potter. It's true that there are a lot of stories out there for the fandom mislabeled as High School AUs, but this is because those typically writing said stories only know the regular high school stereotype. They don't know about non-traditional high schools, nor do they know that the education stages vary from country to country.

Thus if one is to write for the fandom they should be aiming for the College AU instead. They put this option in to make sure "all fandoms are welcome".

AU doesn't mean changing whatever you want.

One of my favorite quotes from wikipedia on Fanfic AUs is this.

"A common mistake made by inexperienced fanfiction writers s to believe that writing an AU fanfiction means that the writer can acceptably and drastically alter the personalities of major characters; in fact, the point of AU fanfiction is that the characters' personalities remain as much the same as possible, and the only changes are those that could rationally be caused by the differences from canon."

The change of a characters personality is one of the biggest issues when people write AUs. This isn't to say some change is expected, but there needs to be some rational as to why the change occurs. For the High School AU the personalities must remain exactly the same. One can strip supernatural elements if they want from fandoms like Lord of the Rings, but they still need to keep the personalities in tact.

The other thing people shouldn't change without some form of rationality is the relationships canon characters have with each other. You can add them, or change them as the plot dictates, but other then that it should stay the same. The High School AU focuses on setting, thus relationships shouldn't be changing at all.