I want to ask a guy to a formal at my school. I like him, but I don't want him to know that and I will do it over text. How do I do it?

Have you spoken to this guy before? If so you could just casually invite him, just mention that there is a formal at your school and ask him if he fancied coming along. That way you're keeping it cool, but letting him know that you want to spend time with him, but not making it obvious that it's in a romantic kind of way, whilst at the same time you aren't coming across too strong. Good luck!

Look, you either like him or you don't. So ask him and see what he says. Stop pussyfooting around with all this "I don't want him to know" bullish!t.

why not just ask him face to face

really? you want to ask a guy out to a formal but you don't want him to know you like him. Hmmm If a girl asked me out I would hope they liked me. if they didn't then why would I want to go? So why try to hide that? And to be honest texting an invitation to me is the wrong way to go. Call him and have a conversation with him. let him know you would like for the two of you to go together. Nothing wrong with showing someone you do like them. Being to shy or playing hard to get is way over rated. It to me is playing games instead of going after what you really want.
Good luck and enjoy the formal.