YA fiction, boy doubles up on high school classes to graduate early with help from magical pyramid

YA fiction book or short story, probably read in 70's, involving teen boy who is given two separate registrations for high school, under his name and the inverse thereof. He decides to take both schedules, merge at the end of senior year, and graduate after two years. His friend has a magical pyramid on the beach (?!) that becomes the popular place to be for the high school scene.

The Mark of Conte by Sonia Levitin

Oh my gosh - thank you SO much. Been telling my kids about it for years, couldn't remember for the life of me!

Thanks,
theresa