Even if they can speak English they won't because they are just a Nong.
In the Thai family I saw one girl for about 3 years she didn't say a word. She never talks when the parents are around. Then one day we went to the store together and she spoke to me in perfect English.
The same thing at school. They won't talk around the Thai-English teachers but they don't stop talking with the foreign English teachers.
The truth is the special program and EP students can speak way way better English then the Thai teachers. The students are near fluent but the Thai teachers sound like farmers.