At the end of the series, Relena steps up to claim the honorary position of Vice Foreign Minister in her father's place. Of course, being an "honorary" position, she was only really hired to maintain appearances — a smart move on the part of the Earth Sphere Unified Nations, seeing as she was the figurehead for peace during the war.
However, it seems that by the time Endless Waltz rolls around, Relena had taken on the real responsibilities of the job instead of just the title. She works to solve post-war conflicts between the Earth and the colonies because she has the power and wisdom to do it. She's a symbol of peace even after the Sanc Kingdom collapses once again. It seems she has quite the influence over politicians — whether it's youth or optimism is unknown.
Relena takes on all these responsibilities at the tender young age of sixteen. I'd imagine, having an important job in politics and mediating disputes between senile old diplomats, she's too busy to go back to school. :P