When I first moved here about 6 years ago, there were a ton of ways to get long-term visas to stay in Thailand year by year instead of maneuvering your way through the maze that is the various other types of visas that exist here. For tourist visas, you used to be able to ride those indefinitely if you were willing to spend a night in Malaysia or Laos, but those are a thing of the past. There were also volunteer visas but because of abuse and people not actually volunteering for anything but rather paying a foundation a certain amount of money to certify that they were volunteering when they were not, those were cracked down on in the past 6 months and are now all but non-existent. There was one type of visa that it seemed almost everyone I knew was on though regardless of their age and that was the language center education visa or Non-Immigrant E visa
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I had one of these at one point and after talking to a load of other people that have had them the actual education part of this education visa wasn't really a thing. My school didn't care if you attended or not and most of them were the same way. In most of the schools that I either witnessed or heard about, the teachers really didn't expect anyone to turn up and much of the time they didn't even have lessons planned let alone a year-long curriculum. Most of the people I know that went to these schools couldn't speak any Thai after months of "education" and I was almost as bad.
The schools were a scam and even though they will deny it, Immigration was aware of this and was likely in on it and getting kickbacks because of it. For around $1000 a year you could stay in the country and not have to worry about visas. When your first education visa expired, you simply signed up for the school for another year. You were able to pull this off for 3 years and then, you simply went and studied something else like Muay Thai or even Thai cooking. All of these were, for the most part, scams meant to circumvent immigration law.
There was a rather massive crackdown on these sorts of visas in the past few years and almost all language schools have gone out of business because of it. There are a few here and there but they are not allowed to offer visas and therefore, the only people that sign up are genuinely interested in learning Thai.
Something emerged from all of this though and it is a fully legitimate education visa that is rock-solid, and not at all dodgy. However, you better be prepared because this is a proper education system with real professors, in real classrooms, with real expectations of you actually learning the language.
There are locations all over the country but the one that is done here locally takes place at Chiang Mai University. You are enrolled the same as any other university student and you are required to complete the course material. I don't think that attendance is mandatory but just like skipping too many classes in university in any subject, you are seriously going to struggle if you do not attend.
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Just like any college course, you meet for many hours a week, you will have homework assigned, and you will have tests that if you do not pass them, you will fail the course. This program hasn't been around for very long but from what I have been told if you do fail the class, your attendance will be scrutinized by both the university and immigration. If you failed but actually have been doing the coursework then there is a decent chance they will let you repeat the same class for another semester. I don't think they'll let you do this indefinitely though like the language centers did for a very long time.
A friend of mine is currently enrolled in the university and he is taking it very seriously. It has been kind of remarkable about how he has, after just one semester, surpassed my ability at speaking the language from me just self-learning over the course of 6 years. So if you are genuinely interested in becoming a real part of the country, this could be the ticket.
University fees are not crazy here like they are in the west so even as a foreign student attending full-time you can expect the total cost to be around $1500 a year including all course materials. This is a pretty good deal if you ask me and if somehow I end up wanting to stay in the country after my work permit is finished, it is something I think I might pursue myself. If you are interested in investigating the program at CMU, the English version of their website is HERE