No one says "at high school". It's always "in high school".
You were heard!
The proposed answer ''Matěj has been in high school for six years.'' is missing a translation of ''už'', isn't it?! Or does the 'has been'-construction include that?
''Matěj has been attending high school for six years'' acceptable?
I have the same question.
je na vs. navštěvuje/chodí na
Why does "střední škola" is translated as "high school"? Shouldn't it be something like "middle school"?
Only if you just translate the individual words without caring what they mean.