Thailand isn’t notorious for having great English. It might have the worst English I’ve encountered so far in SE Asia. That’s why I’m here, to help with native speaker listening.
One of the most obvious things is how the kids pronounce (and simultaneously don’t pronounce) their S’s.
Because of the way that Thai language itself is written, each letter, whether there’s a written vowel or not, has a vowel associated with it. So when the kids look at an English word transliterated into Thai, they automatically add a syllable. Plus, Thai and English have different vocalizations; so, for example, instead of “sssss” they’ll say “sa”. And at the end of words, their “s” sound turns into a non-plosive “t”. So the word “first” turns into “firt.”
There’s more linguistically to this but I’m too tired to look it up now. But trust me, I’ve spent a lot of time this semester holding out a long “ssssss” and holding my hands up to say “superman s” so that the kids know when to make the sound. Only problem is, that some kids will say “sa-chool” or “sa-wimming” because they know it makes me crazy. But at least they’ll realize the difference and overexaggerating it now will make it more memorable and easier to do when they’re adults.