What is the prefix of a student continuing his education after high school and college?

There isn't one single prefix. The prefix depends on the level of education. Some options include:

* Post-doctoral: For someone continuing their education after receiving a doctorate.

* Post-graduate: A general term for someone continuing education after a bachelor's or master's degree.

* Graduate: Used to refer to someone pursuing a master's or doctoral degree.

There's no prefix specifically for someone continuing after high school *and* college.