Metacognition.
And also, I think one really important thing I figured out was that school is a system with certain rules, and if you learn how to play them, you will get ahead. At the same time, you shouldn’t take it too seriously, because grades, etc are just one way of measuring your ability… the ability to play the game.
I would say that my critical thinking skills really got honed in college, not high school. High school was really learning a time to reflect about myself, and the people around me.
Finally, I wouldn’t really call this a skill, but if my students emerge out of high school (still) curious to learn more about the world around them, as a teacher I would consider my work well done.