Though Cillian Murphy takes top billing and turns in a visceral effort in the title role, this is really a formidable ensemble effort as he leads a small group to teachers in what seems to be a constant rearguard action with some of the most unruly teenagers you’ll ever see. Helped out by his deputy “Amanda” (Tracey Ullman) and self-medicating regularly, he has the added pressure of a local BBC television crew…