Gerald Strickland was the principal of Hill Valley High School from 1955 to at least 1985. Although basically a good man, he was a very strict authoritarian who did not tolerate students with anything even resembling an attitude problem, such as Biff Tannen, and even students who he felt weren't living up to their potential, such as George McFly. These types of problem students he collectively referred to, with contempt, as "slackers." He had a particular dislike of George McFly's son, Marty, who he considered the epitome of all slackers.
In the alternate 1985 timeline created when the old Biff from 2015 gave Gray's Sports Almanac to his teenaged self in 1955, the high school was burned down and Principal Strickland become a gun-toting vigilante of sorts, exchanging gunfire with roaming gangs of "slackers" who enjoyed shooting up his house. (He apparently lived just down the street from the school, although it is not known if he lived there in the normal timeline.)