Exposure to the English language in Thai schools is variable at best, unless parents can afford to send their children to more expensive education establishments. For poorer Thais (the majority), while there may be some English taught, it is often not an integral part of the curriculum, the standard poor and while children may be able to repeat a set of standard words and phrases, they probably don’t really understand them.
The further away from large cities, like Bangkok or Chiang Mai, the less English you will find in the schools. The net result is very poor English skills in general but the middle classes will have some English language skills. The problem is exacerbated by teachers giving their students pass grades, no matter what their score.