Nick Cannon is heading back to school after enrolling at Howard University in Washington, D.C. The actor, rapper and TV presenter took to Instagram on Sunday to announce his new endeavor by sharing a photo of himself posing beside a sign for the historically black college. “YOU KNOW!!!!!,” he captioned the shot, in which he was shown carrying a designer Louis Vuitton backpack. “#HU2020 My First Day as a Freshman!!! Howard University Class of 2020.”   YOU KNOW!!!!! #HU2020 My First Day as a Freshman!!! Howard University Class of 2020 A photo posted by Nick Cannon (@nickcannon) on Aug 20, 2016 at 1:05pm PDT Mariah Carey‘s ex-husband has also revealed…