We need more American films like "Up the Down Staircase." We need more films that might be concerned, even remotely, with real experiences that might once have happened to real people. And we need more actresses like Sandy Dennis, who looks as if she may be alive and not a plastic robot turned out by the little elves who constructed Doris Day and Sandra Dee. Here, at last, is a film made in America by Americans in which no one is murdered by a cigarette lighter.