(Read minz's post first, on how "students learn nothing about the methodology and principles of whichever discipline they have chosen, nor the history of the development of said discipline as discipline.")

Further thoughts. Principles are not taught in grade school for 3 reasons.

1) The argument you always hear from the teachers:
Content must precede context. However, by the time my kids have enough content for me to start dealing with context, they are 18 and have graduated.

2) Teachers don't know how.
--> and most (I believe) just think it's a bloody waste of time.

3) All roads lead to "What is the meaning of life?"

e.g.

Q: Why do we study psychology?
A: To understand the mechanisms and causative relationships associated with cognition and behavior, and subsequently be able to make predictions about these phenomena.

Q: Why do we want to make predictions about these phenomena?
A: So that people can use this information to better the quality of human life, in material, or non-material ways.

Q: Why do we want to better the quality of human life?
A: [insert your personal MEANING OF LIFE answer here]

and similarly for all other disciplines. And it's hard to go down that road, and people just don't want to. Better to pretend it doesn't exist at all.