An ex-NFL cheerleader has just pled guilty to raping a 15-year-old boy she spotted on Instagram ... and now could face up to 15 years in prison. As we previously reported, 48-year-old Molly Shattuck -- who cheered for the Baltimore Ravens in '05 -- was indicted back in November for 2 counts of 3rd…

Molly Shattuck -- the 47-year-old former cheerleader accused of raping a 15-year-old boy -- has been barred from the alleged victim's school ... which is the same school her 3 kids attend. McDonogh School Headmaster Charles W. Britton addressed the scandal in a letter to parents, saying he…