There have been several player. Otis Sistrunk of Oakland came to the NFL straight out of prison.. The most recent I can recallwas Eris Swan, a DT for Arizona. You do not have to attend college for Four years. There are many underclassman in the Draft.
Otis Sistrunk was never in prison. He went from high school into the Marine Corp and after serving in the marines he played semi pro in the Continental Football League in 1969. After 3 years in that league ahe was scouted and signed by the Raiders.
In 1952, for the San Francisco 49ers, right out of San Diego High school in San Diego, Charley Powell, 19 years old, the brother of former Oakland Raider Art Powell, sacked Bobby Layne 10 times in the first game Powell ever played in the NFL. The year before Layne led the Detroit Lions to a world championship. Google Charley Powell and you can read about his incredible athletic career. He played minor league baseball. The Harlem Globe Trotters wanted him to play for them. He was a word class sprinter. He was a pro boxer who fought Muhammad Ali-then Cassius Clay in 1963. He was without a doubt one of the greatest athletes whoever lived.