>jesteśmy w 3 klasie szkoły sredniej - we're in the third year (~third
>year students) at high school
Actually, in the U.S. it's called "junior year."
(1 = freshman, 2 = sophomore, 3 = junior, 4 = senior; this applies to high school and college/university)
So I'd say:
We are [ENTER NAME OF THE SCHOOL HERE] juniors.
Here's an example (you may hear from Al Bundy :-) ):
I once scored four touch-downs in one game during my junior year at Polk High.