Impediments

It isn't easy to give a good performance while hollering through a bullhorn, but it can be done. And Morgan Freeman manages it in ''Lean on Me,'' in which he plays Joe Clark, the controversial high-school principal from Paterson, N.J. Mr. Freeman marches through the film at full blast, berating the students of Eastside High School in the tough, bombastic style for which Mr. Clark became well known. The film never wonders what it is in Mr. Clark's makeup that makes him rely almost exclusively on such bullying tactics, preferring instead to admire him as a galvanizing force.

Compared with Jaime Escalante, the mathematics teacher who goads his students into passing calculus in ''Stand and Deliver,'' this film's Joe Clark takes a far less delicate approach. His first act at Eastside, after shouting down an auditorium full of rowdies at a school assembly, is summarily to expel 300 miscreants on the grounds that they probably won't graduate anyway. Mr. Clark, clearly not a man afraid of making enemies, doesn't stop at this. He browbeats his staff, he wields a baseball bat, he padlocks school doors in violation of fire laws (to keep out drug dealers) and he insists that all Eastside students be ready to sing the school alma mater on demand.

Audiences are apt to root for the film's Mr. Clark even when they aren't entirely enthusiastic about what he's doing. Much of this is attributable to Mr. Freeman's fiery and compelling performance, but a lot of it also comes from the director John G. Avildsen (''Rocky''), who has stacked the deck in every way he can. Though the screenplay abounds in one-dimensional characters - not the least of them Mr. Clark himself -and doesn't give much detail to the story of Eastside's transformation, the set design is loud and clear. The garbage and graffiti of the film's early scenes give way to tidy classrooms and waxed floors as the school is meant to improve.

None of ''Lean on Me,'' which opens today at the Sutton and other theaters, is appreciably more subtle than this. In fact, the film's grandstanding reaches such a fever pitch that Joe Clark, wearing heroic white, winds up at the head of a mob of a thousand students chanting his praises, while his chief enemy cowers at stage right. Fortunately, Mr. Freeman himself is not the grandstanding type, and in this scene he avoids gloating and attempts nothing more than a small, ingenuous smile of satisfaction. He humanizes Joe Clark even when the film works overtime to make him larger than life.

Also in the cast are Robert Guillaume, as the school superintendant who gives Mr. Clark his chance to resurrect Eastside; Beverly Todd as Mr. Clark's loyal but underappreciated assistant principal; Jermaine Hopkins as a kid with a drug problem who is grudgingly given a second chance by Mr. Clark, and Karen Malina White and Regina Taylor as a student and her mother who are put back on the right track thanks to Mr. Clark's