The true Princess of Equestria.

Now they’re sisters, and they will rule all!
 

And the next day she got her cutie mark in leading a revolution against unjust summer homework. It was a not-so-bloody battle when Diamond convinced Cheerilee how much more free time she’d have over the summer and how most fillies and colts never came back any smarter for having done their homework anyways. (which most never did it anyways but Cheerilee didn’t need to be reminded of that particular detail.)
 
And thus, with victory at hoof and three sheets less of homework each week to boot, the butt did shine’eth its mark of destiny upon her flanks.
 
Many milkshakes were had that day.

Headcanon: Diamond Tiara is destined to become an alicorn princess someday. She has a LONG way to go, though.