Mario Lopez, Tiffani Thiessen Share Touching Tributes to 'Saved By the Bell' Co-Star Dennis Haskins

Mario Lopez and Tiffani Thiessen are remembering their on-screen principal, Dennis Haskins, with touching tributes after the "Saved By The Bell" star's death.

Mario -- who played A.C. Slater on "Saved By The Bell" -- says he's "heartbroken to hear about my old friend Dennis Haskins."

He continued ... "If you knew him, you loved him -- and if there was karaoke, you knew Dennis was getting on that mic!"

Mario added ... "I'm grateful for the laughs, the memories, and the friendship we shared all these years. Rest easy, my friend. You'll always be part of our Bayside family."

Tiffani -- who played Kelly Kapowski alongside Dennis' Mr. B -- said he was a steady presence in her teenage years ... and "will always remember his kindness."

Mario and Dennis were both featured in all 86 episodes of the show ... with Tiffani joining then in 76 of those episodes across all 4 seasons from 1989-1992.

The 75-year-old was known for playing Mr. Belding on the popular sitcom, trying to keep the Bayside High kids out of trouble with his catchphrase ... "Hey, hey, hey, what is going on here?"