The Hebrew expression for high school or upper school is בית ספר תיכון, or simply התיכון.
יש לך כבר ילד בתיכון?!
You (a woman) already have a kid in high school?!
If high school is the top of state-mandated education, why does Hebrew use תיכון – a word that indicates something in between, since its root is ת.ו.כ – inside – to describe it?
The answer is unclear, but it may have something to do with בית ספר תיכון being in between lower and higher (college, university) education. The term was coined in 1875, back when the school system was different.