Hi Alexis! As Brandon said, you would almost definitely (if you are in the UK then this is definite) need to take in a letter about your brother’s appointment, usually at least the day before the appointment. The letter would need to be signed by your brother’s parent or legal guardian. If that is not you, then the letter would need to give permission for you to take your brother out of school. In such a case, you probably won’t need to say much! I hope that helps and have a nice day!

There are no specific phrases used in this circumstance, but I believe the usual process requires bringing a written note of some kind. You could say, "Hello, I'm Alexis Espinoza, _________'s brother. I need to check him out early today for an important appointment." I know it's very simple, but that's how it usually goes. I think you'll be fine! If you are not his parent/guardian, you might need the parent/guardian to call the school ahead of time.

*whoops* meant to "answer" instead of "comment"