Answer: Robert Guilluame
By and large Mr. Napier is supportive of Clark, battling even the mayor on behalf of the principal. But he, too, recognizes that Clark steps over the line, and during one argument has to remind his colleague that it is he, and not the principal, who is the head person in charge.
All four of the actors featured in this question have had stellar film and/or television careers. Before his role as Mr. Napier, Guillaume won Emmy Awards as the irascible title character "Benson" and as the sarcastic butler (the same Benson character) in "Soap". Poitier starred in "To Sir With Love" and was the first African-American to win a Best Actor Oscar for his role as Homer Smith in 1963's "Lilies of the Field". Braugher had major roles in "Glory" and TV's "Homicide: Life on the Streets", winning an Emmy Award for his work in the latter. Rhames has starred in many projects, winning a Golden Globe Award for his role in HBO's "Don King: Only in America".