This is the lesson of the Harry Potter books, according to Peter Appelbaum, of Arcadia University in Pennsylvania, who has studied the morally ambiguous role of Severus Snape, the potions master (played by Alan Rickman, above) - and his real hero of the books.

Initially, the books present a dim view of pedagogy. Professor Appelbaum highlights the “unbearable and comic teachers” at Hogwarts. Snape, by contrast, is talented at spells, potions and mind-reading.

But it is not merely his subject knowledge that singles Snape out. While other teachers favour certain pupils, Snape acts as mentor to Harry and his arch-enemy, Draco Malfoy. He is the only adult who influences both pupils equally.

Boys need male teachers to serve as positive role models. And while the books appear to hold up Albus Dumbledore, Hogwart’s headmaster, as an ideal teacher, it is Snape who proves the better role model.

“Dumbledore is revealed as a victim of greed,” says Professor Appelbaum. “Snape acts consistently on more noble motivations, sacrificing his life for the good of humankind.”

There are no websites devoted to Dumbledore, but Snape fan-sites abound. “That’s because he is sexy.”

But Snape’s real strength is his ability to allow Harry to disobey him, so that he can learn from it. “Cultivation of self-will and self-sacrifice for a greater good demands that the apprentice dismiss the wisdom of the teacher,” he says. “Trainers of teachers and educational policy-makers may not want to think about this.”

Photograph: Roland Grant Archive.