High school Drama is the drama between people at highschool, which people seem to create around themselves just to have a bit of action in their lives. Highschool drama is a phrase used to ridicule this. An example is below:
Person One : Oh my god! Shelly got with Brad, even though Brad is seeing Margaret, who by the way was with Jamie when Jamie and Andy were meant to be going to the big game, I cant belive Andy had to go alone, especially after Jamie slagged him off for sleeping with Jess's mother.
Person Two : Who cares about your STUPID HIGH SCHOOL DRAMA?!