Joe knew just what to do when his daughter, who was studying Roman history, came home with an assignment to build a catapult. He ordered a catapult kit from the Internet and put it together himself.

The task, he was sure, was not in his 12-year-old daughter's skill set, and he wanted to give her a hand. His effort earned a D, said Joe, who asked that his last name not be used to avoid antagonizing officials at his daughter's Montgomery County, Md., school.

His daughter's overall grade suffered from Joe's D. But what most annoyed him about the project was that other catapults — clearly built by parents with more expensive kits — got higher grades.

The episode underscores a growing tension over how much parental involvement in daily homework and projects is appropriate and who is to blame when parents cross the line.

Educators say that the correct level of involvement depends on the child and his or her developmental stage but that some parents are so competitive that they go overboard.

"You sometimes run across parents who are what I call Machiavellian parents who are trying to justify the ends any way they can," said Dan Kent, a social studies teacher at Broad Run High School in Loudoun County, Va. "There is such a push to get the kids in the best colleges that sometimes you find a parent who goes over the edge and does the work for the kids, even in high school."

Teachers also say parents don't always understand the lessons a child is supposed to learn from a particular project; making a collage, for example, isn't a waste of time, as some parents contend, but can promote fine-motor, classification and other skills.

But some parents say schools are demanding more parental involvement by giving assignments that children can't do on their own — such as math problems kids don't have a clue how to attack — and failing to draw boundaries about what is acceptable parental involvement.

"I think teachers are wellintentioned, but some of the assignments are impossible for some kids to do," said Marcia Simon, a Montgomery public school parent of two, recalling one assignment in which her elementary school-age son had to design "a very lovely" restaurant menu with pictures and nutritional information about the food.

"For kids who like to do art projects, it is fine, but for kids for whom this is not a strength, it was torture," she said.