תּוֹךְ (tókh, “inside”) + ־וֹן (-ón). Compare רִאשׁוֹן (rishón, “first”) from רֹאשׁ (rósh, “head”). The sense high school is an ellipsis of בֵּית סֵפֶר תִּיכוֹן (“high school”), and the sense median is a semantic loan from English.