Sleep on the angry message. Morning-you reads it with different eyes.

Anger is a narrowing. It makes one sentence the most important thing that has ever happened, and it makes a reply feel urgent in a way that will embarrass you by breakfast. Nothing about the situation changes overnight, but the person reading it does, and that person has access to proportion.

Write the whole furious thing if you need to, and put it somewhere that cannot send. A note on your phone works fine. Read it before school and keep whatever still seems true, which is usually about a third of it and almost never the worst line.