Frederick K.C. Price III Christian School in south Los Angeles was set to shut its doors for good this week after it lost the backing from a local church -- until the Brooklyn Nets star caught wind of it.

Crabbe -- who attended the prestigious inner-city school from the time he was in diapers to when he graduated some 18 years later -- agreed to supply the funding to keep the school ticking.

Unclear how much he actually kicked in for the cause ... but sources tell TMZ Sports it was a substantial 6-figure check.

“I was at that school practically my whole life,” Allen says. “I’m just happy and feel blessed to be in a position now to do this.”

Thanks to Crabbe ... 175 kids don't have to find a new school next year -- and school officials are calling it a "miracle."