One of the ways that students can improve their critical thinking on IB exams is to receive coaching along the way. It is often the case that students get feedback after completing an exam or a take-home essay in which they are told that they did not use appropriate strategies, enough strategies or precise vocabulary. Often this essay then gets filed away and the feedback is not consulted again before leaping into the next assessment task.
Another way to give feedback is during the task. Word walls are a good way to do this.
Word walls are simply visual representations of information for students while they are carrying out a task. This could work in many ways.
One way is to have the words necessary for critical thinking projected or written on the board in the classroom during the assessment. I wrote the words and then hide them behind the screen. When the students begin to write their essay, I then pull up the screen so that they can see the words. They are not allowed to talk to me about the words or ask questions, but this is to get them to remember the different ways that have applied critical thinking during the course.
Another step is to then have them underline in red pen any term that they use from the list. In walking around during the assessment, give them direct feedback on those terms. It could be - have you linked this to the study? Or - "this is a good application of the term. Try to use some of the other terms now." This kind of feedback during the assessment helps students to develop critical thinking strategies on the spot and helps them to improve overall on the skill. As they get better at it, you can take this scaffolding technique away and they can stand on their own.