College Football Player -- Death Threat Was 'Just a Prank' ... Says Player's Dad

The Fresno State football player who was arrested after posting about carrying out a school shooting was just "fooling around" and didn't actually plan to do it ... so says his father.

18-year-old Christian Malik Pryor was arrested Monday after posting a picture of the FSU campus to Yik Yak with a caption that read, "the time is here. @3PM I will release my frustrations. Tired of dirty looks, get rejected, nd being talked about bc how I dress. My choice of weapon M4 Carbine."

Pryor added, "I will take a headshot at a hot blonde."

But Pryor's father, Gene Hopkins, tells TMZ Sports he has spoken with his son -- and insists the whole thing has been "blown out of proportion."

"That was just a prank that got out of hand," Gene says ... "He was trying to delete it and it got posted accidentally."

"Christian's a good kid, good athlete, a student. He's never been in any trouble, he doesn't own any weapons, I think it was just a Halloween prank that went bad."

"I asked him why he did it and he couldn't come up with a reason. He said he was just fooling around."

"I talked to him a couple times yesterday. He said, 'I don't know what I was thinking.' I think there's more to the story than we know."

Pryor is facing a felony charge of making a criminal threat -- and if convicted faces up to 3 years in prison.