Excluding what you learned at home, what was the first scientific principle you remember having learned at school? ~

Archimedes principle.

Newton's Law

Atoms.

I think it was our Primary School's fourth-year form teacher demonstrating the basic phenomenae of refraction using glass-blocks on a board.

He didn't introduce the mathematics as trigonometry would have been a couple of years in our future in senior school, just showed us the behaviour of a ray of light through the glass. In fact marrying the refracted ray to sines and cosines was even further away, as a Sixth-Form, Advanced-Level Physics topic!

As far as I know this was not part of the curriculum ending in the 11-Plus Examination, but I think teachers at the time (early 1960s) had a freer hand and opportunity to add such things.

I learned the principle that adults were idiots and kids are too.