“The setting is a ballet class, and the year is 1974. George Balanchine throws up his arms in exasperation at the sight of a dancer executing a step incorrectly at the barre. We may not be able to see her, and what she’s doing wrong, but we feel how hard Balanchine is taking it. It’s not just his words — ‘that’s bad’ — but the punctuation of his body, emphatic, agile, alive. His hands slap his thighs. He raises an arm like a stiff branch to show how far a leg should be raised. It’s not high; it’s parallel to the floor. … The new film ‘In Balanchine’s Classroom,’ directed by Connie Hochman, focuses on the teaching of the groundbreaking choreographer — and how it instilled his dances at New York City Ballet with articulate, musical brilliance. It’s both enthralling and heartbreaking. To love Balanchine is to love this film…