Cardi B really believes in public education … because she went back to her old middle school in New York for a surprise visit, and even whipped out her checkbook.

The Bronx-born MC hit up I.S. 232 Tuesday morning in NYC’s Morris Heights neighborhood. Cardi did 3 years there — 6th grade through 8th — and we’re told her visit came as a complete surprise to the students and principal.

The unannounced drop-in included Cardi having an open conversation with students about their favorite part of school, favorite teacher and some sage life advice.

Cardi also answered a couple of the students’ questions … and at the end of her visit, she presented I.S. 232 with a $100,000 donation which she wants to see go towards after-school programs, including tutoring and music/dance.

The students let out a huge cheer when Cardi announced the donation, while teachers and administrators almost started crying.