Your voice will crack the first time you tell a bully to back off. Cracked still gets heard.

People wait until they can say it steadily, which in practice means waiting forever, because a steady voice is a thing that comes after doing it a few times rather than before. The sentence works regardless of how it comes out. A crack does not cancel the content, and to anybody listening it often lands harder, because it is obvious you said it anyway.

Keep it short and then walk, rather than staying to win an argument. Afterwards, tell an adult who can do something. You should not be handling this on your own.