After quitting school to pursue acting as a teen, the star has taken classes with tutors on movie sets.
The 19 year old finally qualified between scenes on the Vancouver, Canada set of Eclipse, at exactly the same time as her character Bella - so the fictional graduation ceremony she shot for the film doubled up as her real one.
In a chat with Dennis Hopper for the latest issue of Interview magazine, Stewart reveals, "I know it's a little late, and I'm a little old, but I just finished high school - with honours.
"The other day I was doing a graduation scene on Eclipse and I had just finished high school myself the week before, so I told the crew, 'Hey, just so you know, I'm actually graduating right now, and I'm not going to have another ceremony.'
"So I took a mock picture with an extra. I literally asked the actor to come back and shake my hand and hand me the diploma while I was dressed in a cap and gown."