* Elementary/High School: Children automatically attend school when they reach the appropriate age. You don't need to enroll them; it's handled automatically by the game. They'll leave for school in the morning and return in the afternoon. Their grades are tracked, but there isn't a specific prep school option that impacts their future college prospects beyond grades.
* University: If you have the "Discover University" Game Pack, teens and young adults can attend university. This is the closest equivalent to a prep school experience in the sense that it's a higher level of education before entering the adult workforce, but it's not a direct replacement for what a real-world prep school might represent. University is an explicitly chosen option and requires enrollment.
In short, there's no in-game mechanic to specifically send kids to a prep school. Their academic progress is managed through their automatic school attendance and, later, the optional university system.