EXCLUSIVE

"Big Brother" contestant Donny Thompson just learned a hard but basic lesson ... when you don't show up to work, you get fired.

Donny -- who's been the groundskeeper for Cabarrus County School District in NC for 6 1/2 years -- took a 2-month vacation when he was cast ... but he did so well on the show he stayed past that.

Donny returned home to no job ... he was fired. But his position had not been filled, so he reapplied ... assuming he'd have a lock on the job.

Another guy got the job, but Donny wasn't left totally out in the cold. He eventually got rehired -- but as a part-time janitor.