Apple CEO Steve Jobs slammed US teachers' unions last week, saying that unions had become too strong in their defence of bad teachers.

In an unusual tirade — the second in so many weeks — Jobs argued that US school principals are unable to fire bad teachers.

He observed that no matter how much technology is pumped into classrooms, these tools are of no use in the hands of weak teacher.

Associated Press reports that Jobs was sharing the stage with Dell CEO Michael Dell. Both men were discussing their vision of how digital technologies can boost educational achievement.

"I believe that what is wrong with our schools in this nation is that they have become unionised in the worst possible way," Jobs said. "This unionisation and lifetime employment of K-12 teachers is off-the-charts crazy."

Jobs added that he was sure "Apple just lost some business in this state," as a result of his statement.

The Apple boss also talked up his vision of textbook-free schools in which all educational materials were made available in digital formats. He proposed that this would mean more resources would be available for teachers and pupils and observed that such a move may free budgets for other uses.

Dell was more pragmatic. Rather than slating bad teachers, he pointed the finger at bad principals, a problem he blamed on the lack of a competitive job market for school chiefs.

Dell also pointed out that teachers' unions emerged only because employers treated teachers badly.