My students don't speak English

I love my students, I really do. But I've come to the conclusion that, with just a very few exceptions, they do not speak English. Three Matayom 6 classes in a row failed to conjugate the verb “be” in an exercise on my worksheet – even the smartest kids didn't manage it without prompting. I really shouldn't have to be going over the verb “be” with M6! Mind you, when I was observing in a couple of lessons with Thai teachers the other day it's easy to see why – they barely spoke a word of English the whole time I was there except to read the example sentences from the worksheet.

They're taught about English, presumably on the basis that if you know enough theory about a language, you can speak it. But it only serves to confuse them – and to be fair I can hardly blame them: to come from a language with no tenses to be introduced to the past pluperfect possessive progressive obsessive compulsive third subjunctive, it's gonna confuse 'em! Which is why I end up with M6s coming out with the construction “they are go” and “the sea is shine”. After being bombarded with grammar for six years, they can't actually construct a sentence. If you set them one-word exercises to conjugate verbs in the past continuous they could probably do it, but they can't deal with having to think. Surely the emphasis ought to be on communication? But almost to a one they're unable to hold even a simple conversation. I speak slowly, clearly and precisely, but even M6 students didn't understand “do you want to go to university” until I pronounced it with a Thai accent. Frankly, if they can't understand the way I speak, then they can't understand English. And yet these kids are going to leave school in under a month, with a qualification in English.