Johnny Manziel: I Promised My Parents I'd Graduate So I'm Goin' Back To School

Johnny Manziel vowed to his mom and dad that he'd graduate from college ... and that's why he re-enrolled at Texas A&M ... the former NFL QB tells TMZ Sports.

We asked Johnny -- who left college after 3 years to enter the NFL draft in '14 -- about reports he'd signed up for classes at his alma mater.

Johnny confirmed he has already started class ... saying he "enrolled not too long ago."

But why go back?

"I'm trying to get back to my roots. I need to get my degree like I told my parents I would," Manziel told us.

No word on the how many classes J.M. needs to graduate ... or if attending school will affect his pursuit of a return to the NFL.