I couldn't find your Benjamin anywhere in NC in 1880. I believe this is him. He was the only Benjamin Knox in Phillips County both in 1880 and 1900.
1880 Census L'Anguille, Phillips Co., AR ?(Student in "Southland College") ?Benjamon Knox B M 20 single At School b. AR Parents birth places left blank.

I couldn't find Samuel in 1880.

1900 Census Cleburne, Phillips Co., AR ?Benjamin W Knock Head B M b. July 1862 age 37 married 11 yrs b. NC Father NC Mother NC Occu: ??????? ?Henrietta Knock Wife B F b. Aug 1866 age 33 married 11 yrs 4 children born 2 living Occu: House Keeper ?Elnore (Elmore?) Knock Daughter B F b. July 1888 age 11 single b. AR Father NC Mother NC Occu: AT School ?Effie Knox Daughter B F b. Apr 1892 age 8 single b. AR father NC Mother NC Occu: AT School
next-door
Samuel L Knocks Head B M b. Sept 1861 age 38 single b. NC father NC Mother NC Occu: Farmer
For more information about Southland College:
http://www.google.com/search?hl=en&q=%22southland+college%22+phillips+ar
Also
www.cals.lib.ar.us/butlercenter/abho/docs/Southland%20College%20history.pdf -