why are most anime plots take places at a school?

Most animes are aimed at teenagers. So they have teenage characters that teen viewers can identify with. Teens want to watch/read/hear stories about other teens.

Most anime/manga is targeted at kids 10-18...so, school is going to be a pretty common setting.

1. Most of the target demographics were also in schools (shounen/shoujo)

2. In Japan (and some other countries too), schools, especially high schools, is considered as the "primetime of youth" where lots of memories and life stories were made during that time. Even some of the titles were so down-to-earth, it depicts the real daily life of a highschoolers there.

So yeah, it's basically a matter of the target readers and cultural thingy. If you want some comparison, it's similar with the Archie comics on the west.

That's the demographic that has the most free time and is the most likely to watch anime.

Since the anime is mostly teenage based, as smarty said, it would naturally happen that many of them would either be school based or have school scenes in them. This is not always the case, as I have seen many that are not school based at all, even though the characters were school aged.

There is also quite a few anime titles out there where school is never part of the story.

That's only the "slice of life" genre (normally about teens falling in love, and other teen-age problems) there are many anime that doesn't have anything to do with school.