(n./adj.) This phrase can be used to describe people that routinely bring up high school when they are well into their 30's, because they haven't had a single exciting or interesting experience since their senior year, and as such their lives and themselves are said to have, "peaked in high school."
This term is often used when speaking disparagingly about an individual or group. This phrase can only be used appropriately after-the-fact (i.e. after individual/group leaves high school) to accurately gauge if (s)he qualifies for such status.
I have noticed a lot of athletes, clowns, and bullies peaked in high school, whereas geeks, nerds, and dweebs are often at the lowest risk of 'peaking', because their high school experience(s) leave them no where to go but up.