What should I expect when re-entering Thailand on an ED visa for Thai language studies after traveling abroad?

Suggest you read this recent post about Ed Visa and DENIED entry at the airport.

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My advice... DO NOT LEAVE THAILAND WHEN YOU HAVE AN ED VISA OR EXTENSION IN YOUR PASSPORT

I had a few problems, my advice would be to stay calm and be polite throughout.

𝗛𝗮𝘀 𝗮𝗻𝘆𝗼𝗻𝗲 𝘄𝗶𝘁𝗵 𝗮𝗻 𝗘𝗗 𝘃𝗶𝘀𝗮 𝗳𝗼𝗿 𝗧𝗵𝗮𝗶 𝗹𝗮𝗻𝗴𝘂𝗮𝗴𝗲 𝗲𝘃𝗲𝗿 𝗵𝗮𝗱 𝗽𝗿𝗼𝗯𝗹𝗲𝗺𝘀 𝗿𝗲-𝗲𝗻𝘁𝗲𝗿𝗶𝗻𝗴 𝗧𝗵𝗮𝗶𝗹𝗮𝗻𝗱

Yep many MANY people have had issues re-entering the country on ED visa extensions.

I would say you do NOT have a visa that's valid for a year, you have a stamp in your passport that says extension of stay and has a date.

Even with a letter from your school saying they know you will be leaving the country and it's okay, you can expect to be grilled about why you left, when you come back.

Plenty of people on valid ED visa extensions from private schools <- the ones who get extensions every 90 days had problems re-entering the country.

There is no recommendation, other than have the letter from your school AND have a contact number for someone at the school that can pick up the phone if you get stopped at passport control.

It appears that the Thai immigration officers at passport control are of the mind you should be IN country IN school, not traveling around..

Good luck with it,