I want to ask my ex not to show up during the first week of school, since we broke up over the summer.
That's a big ask of someone to make. I'm pretty sure he's required to be there.
I don't care but I just hate the presence of my ex at the first week of school
What you are asking for is selfish, and your ex has no obligation to follow it. Just focus on your own life, not your ex's.
I tell you what, who gives a fuck about other people.
Would you expect your ex to ask you to skip an entire week of school? Think about how that would feel. You should care about him more than that even if you're not together.