The problem though with critical thinking is that it encourages students to challenge their teachers, and when you have standardized curricula and testing then you can't really challenge what is going to be considered "fact" on the testing. This is why critical thinking isn't really encouraged until post-secondary, when challenging your professors is part of what will give you a good grade.
Let me put it this way: let's pretend that I'm writing a Biology paper, and it comes up with a question asking about the function of adrenaline. Let's say that I recently read a peer-reviewed journal article that says it doesn't directly affect your sympathetic nervous system at all and they have studies that prove this, and that stimulation of the sympathetic nervous system is actually caused by something that is almost always secreted simultaneously as adrenaline (this is a huge hypothetical, and probably could be disproven rather easily, but for the case of this example let's pretend it's so).
So final exam, question saying "Explain the function of adrenalin" and you spend a two-page essay theorizing its other uses based on this study and looking towards other evidence gathered in support of the adrenalin-stimulates-sympathetic theory.
In High School, this would get you an 0, because the teachers marking it have to mark papers for ~1000 students and they don't have the time to verify your studying, and their standardized key shows just that it stimulates the sympathetic nervous system.
In secondary school, however, your TA only has about 150 papers to mark, and so they'll do some minor fact-checking, realize that you've now discovered this original and revolutionary theory, and quite probably give you an A.
The thing is, science isn't exact and it never is. However, so that we can actually grade the masses on their understanding of what we're pretty sure we know so far, we pretty much treat it like it is in high school.
High school isn't designed to make you think, it's designed to teach you what we think we know. The value in a post-secondary education is that it shows that you know how to actually understand and challenge a topic as opposed to just regurgitating learned information. Teaching critical thinking in high school would just confuse the majority and devalue a college education.
That being said, if you really think your kids should be learning critical thinking in high school, see if you can get them enrolled in some college-level exams, such as the AP exams. They actually do teach you a lot about critical thinking. I took three AP courses and was insanely thankful I did. Biology was especially interesting, since I got to learn DNA fingerprinting, and got to transform bacteria to become penicillin-resistant. IN HIGH SCHOOL. Also, my provincial exams became a cake-walk because I knew all I had to do for them was just memorize shit, and the _REAL_ tough exams were like a month before my provincials, where I had to critically think about things during the exam. Heck, I even came across a bunch of questions that I didn't even know the answer to before the exam, and I actually had to figure through the question to get the answer. Insanely valuable skills that paid off tenfold in first year.