You've got to keep this really simple for it to work and the first part of this process is to never mention the term ‘Critical Thinking' to your students. The second part is that you're not teaching Critical Thinking itself.
Go Google the subject and see what comes back. Visit any decent academic library and look at the section on it. The world's strongest man couldn't lift that lot.
What I'm presenting here are the essential basics of a 6-step process to help your students expand the English that you're teaching them so that they can use it in more and different ways. All or some of which may help them to retain more of it and actively use it.
The following is a pyramid and you're building it from the bottom up.
Step 1: REMEMBERING. Of course students have to be able to recall verbs, nouns, adjectives, adverbs, etc. You already know how to get them to retain new vocabulary. If you don't, check out my article on games for large unruly classes. If they can work for me in Thailand, then at least some of them should work for you in Thailand, too.
Step 2: UNDERSTANDING. Once students are armed with this vocabulary, the grammatical stage can begin where they can ask and answer questions in set formats and also describe things and situations. Expand this step as much as you feel is necessary.
Step 3: APPLYING. Give students some new vocabulary and have them create their own descriptions and questions & answers of the subject. If they can do this, you're on the road to success. Don't worry if a lot of them falter at first. This is a big unsupported step but they can become accustomed to it given time. Brighter students will quickly realise the fun/value of free thinking and communication, i.e. ‘Mr. Geoff smells!', ‘Mr. Geoff likes Barbie!' ‘Do you like Mr. Geoff?'... ‘No, I don't!'. [I teach P1 to 4 by the way, so you should up the ante for older students. I wouldn't advise using this method in P1. Use that first year to teach students the basics of Q&A and describing things].
Step 4: ANALYSING. As an example, give students the ability to be able to describe and compare certain subjects. This can be anything, animals, food, music, household items, clothes, cartoons, etc. Then have them describe the subject, mentally and in writing. Why and how are things different? What does what? Why does something do something? Etc.
Step 5: EVALUATING. Have students swap their Step 4 masterpieces with other students. Do they agree with each other? Give them the tools to be able to say why or why not. Keep this step lighthearted and don't turn it into a competition or a test. You should support and encourage weaker students across steps 4 and 5.
Step 6: CREATING. Here you can have students collaborate in pairs or groups [make sure that skill levels are balanced though and don't let students self-select]. Have them produce another piece of work similar to Step 4 and then have them swap this amongst other pairs or groups. When universal agreement has been reached without correction or criticism you've pretty much hit the nail on the head.
Bringing basic Critical Thinking to your lesson plans and classroom can help your students to think more actively in English. Something that is almost impossible to achieve if all you do is work your way through the textbooks, most/all of which are in the third person and allow students very little scope to express themselves.
Example, I struggle to see the use of being able to say what others are doing but not what you are doing, have done or intend to do. This must grate on many students nerves by the time they hit secondary but they have no way of telling you that.
There are many reasons why a lot of Thais can understand you or at least get the gist of what you're saying but only respond with ‘yes' or ‘no' and this certainly has to be one of them.
And this is probably one of the reasons why more than a few local English teachers don't stop you in the corridor for a quick chat or sit opposite you over lunch.
Students have got to learn the art of creative, comparative and critical expression alongside the rote mechanics of the English curriculum.
You shouldn't formally test students on their newly acquired Critical Thinking abilities. Let them go with the flow and see how you get on. Don't expect a 100% success rate because you won't get one. If you do, write a book on the subject and have someone like Wiley or Oxford publish it for you. ;o)
As an experiment though, I would love to try this approach as a weekend class with a bunch of the brightest students. The results could be fascinating. My guess is that a bunch of stars would emerge, highly competent in conversational English.
If you want to learn more about the subject itself, there are some good Critical Thinking primers and introductions available through Google. I prefer the plain English to the technical highbrow stuff myself.