Claims He Was Forced To Resign

A volleyball coach is calling out Denver's Valor Christian High School -- claiming the administrators "grilled" and belittled him after they found out he was gay ... and forced him to resign.

Inoke Tonga shared the details on Sunday ... saying he had a meeting with the campus pastor and athletic director last week about a Facebook post he made suggesting he's gay.

"I was asked if I had any changes in lifestyle that were life changing," Tonga said on Instagram.

"As I was asked repeatedly that question, they finally asked if I had posted something inappropriate on my social media that would go against Valor's culture and beliefs."

"Eventually they asked if I did an interview, or posted anything…