What happens if I leave Thailand for a month on a 1-year ED visa for studying Thai?

I just did that 2 months ago and spent around 40 days out of thailand .. everything was super smooth no questions asked except they were making fun of my thai accent 😅

You need to have more than a month left in your last extension and a re entry permit to do so

The ED visa must be extended every 90 days, and if you did not get a re-entry permit before you left Thailand, then as soon as you checked out with immigration, your ED visa ended, and you start over. The ED visa might have been sold to you as a one year visa, but you still must extend it with immigration every 90 days with the document bundle your school gives you showing that you are attending in person regularly, and passing. If you miss one of these required 90 day extensions, then your visa is voided, and no longer valid, which can also mean you are on overstay.

After a few extensions, those officers will speak to you in Thai and expect you to reply them in Thai as well. Else they won't give you extension anymore.

Better be for a very very good reason. As Brandon said you are here to study not travel.

If you have a re-entry visa, yes. If you can't get one, no

You're almost surely going to be pulled aside by immigration and grilled. An ED is for studying in person. You can't do that outside of the country, especially for a whole month. You better have a letter from your school explaining why you're going to be gone and that it's okay as well as your own documentation about the critical reason you need to leave the country like taking care of a sick parent or something. But not even sure if that will help for a whole month.

Also there's no 1 year ED visa to learn Thai.

You get a 90 day ED visa then have to go back to immigration with documentation from the school every 90 days after to apply for an extension along with proof of attendance and may be quizzed in Thai by the immigration officer.