That's false. You can convert a visa exemption stamp to an ED visa inside the country. Whether or not the school will have the time to prepare the ED visa documents in time depends on the speed of the school.
And avoid any school that asks for "visa processing fees" or "visa renewal fees." This is a scam practice that some bad schools do. A visa costs 1,900฿ at immigration to get the ED visa, and 1,900฿ to renew your visa every 3 months. Nothing more.
Get a TR visa in your home country. You will get 2 months and possibility to extend another month inside the country for 1,900฿.
Preparing the ED visa documents can be a lengthy process. Can take a week, or maybe even 1 month for some schools.
Also, I believe you need 15 days remaining on your current visa (21 days in Chiang Mai and a few other provinces), to make the switch to an ED visa. Visa exemption stamp is 30 days + 30 days if you extend, so your school will have little time to prepare your documents. So TR/SETV might be better.
That's not the issue. It's that my passport, when applying for the Thailand Pass, is new, but my vaccine booklet has my old passport number. So I don't know if this will cause issues with getting the Thailand Pass in the first place.
I'll be carrying my old passport, but can't do so for Thailand Pass, or anything like that. I appreciate the effort, but I can't travel internationally based on someone thinking it will probably work. Could be stuck in the middle of nowhere and be deported back to my home country if refused entry.