Eastman speculated kids are attracted to the turtles because of "the family attitude with the four brothers and Splinter playing the father figure and the teacher."

Levin acknowledges the Ninja turtles aren't the first superheroes to beat up on the enemy. But the intense marketing of everything from turtle toys to lunchboxes has provoked an obsession with the creatures among children between the ages of 18 months and 10 years, she said.

Superman had friends, and a life outside crime fighting as his alterego, Clark Kent. "The generic hero, kids could try to emulate," she said.

Not the turtles. "Mostly what they do is fight," she said. "They're very unidimensional."

Some teachers noted that they had used the turtles to try and inspire children to learn about other subjects. One teacher noted she was able to involve more boys in an art project by asking children to make turtle masks.

In some cases, though, trying to link the Ninja turtles to classroom projects backfired. That happened when a teacher brought live turtles to class.

"We have one case of a turtle being killed by being thrown against the wall," Levin said.