I liked a guy for a year and he confessed that he liked me. We agreed not to date yet; we didn't think we were ready for it. He moved schools. Suddenly I find that he has a girlfriend. He never told me. How do I ask him about it?

You don't ....you already know he has a girlfriend , move on then... Cos I said so :)

He lies .. Obviously .. So how can you trust anything he tells you ? 

Considering you already know he has a girlfriend ... No need to contact him about anything .. He's off limits .. Period 

So, here's your Question:

"I liked a guy for a year and he confessed that he liked me. We agreed not to date yet; we didn't think we were ready for it. He moved schools. Suddenly I find that he has a girlfriend. He never told me. How do I ask him about it? "

And Here's what you say to him:

I liked you and confessed that you liked me. We agreed not to date yet because,  we didn't think we were ready for it. After you moved to a different school . . . I find that you have a girlfriend. you never told me. What about us??

I took the liberty of "recycling" the question (Underlined) into the conversation . . .

Feel free to use at your leisure . . .

If you are going to be "Ready to Date", you need to be ready to "Communicate" . . .

You and he obviously had different expectations about your possible future relationship.

Quite understandable.

If you want to follow up, you can---just contact him and chat about you and him and see what he says.

With additional information, you can decide how to proceed if you still want to.