I probably won't have any specific advice, since I've never done anything like that. But, before the advice from others starts rolling in, you need to mention the country you're in. Italy? USA?
How do I host a high school chess tournament?

Oh yeah, I currently live in America lol, sorry.
Since you're in the USA, I'm sure you won't want to mess with FIDE. The real question will be whether you want it to be connected to the USCF or not. I can't really advise you on that, but maybe others can.

You also might want to skim the threads in the "Scholastic Chess" forum:
https://www.chess.com/forum/category/scholastic-chess
Hello, I'm a student in high school whom isn't very good at chess, but I am apart of the chess club. But, my school is part of a district that consists of 5 different high schools, and each has their own chess team, unfortunately, there are no tournaments. However multiple staff have agreed to advocate for one if someone were to start it, and I have experience with hosting events, so I've volunteered to make an attempt. Funding will come from the school.
So, my question is, where do I start? I understand the regular things, venue (most likely one of the school's gyms), FIDE licensed arbiters (I don't even know where to get one), but there are of course other factors. I've tried reading the documents but they're all very confusing to someone without much knowledge in the legal side of chess. So if someone could please explain what to do in layman terms, I will actually love you for forever.
Edit: I currently live in California, USA