Union rules that give preference to veteran teachers are shortchanging students in urban schools, a new study contends.

As many as 40% of teachers are hired by principals who have little or no choice in the matter, according to a report released yesterday by the New Teacher Project.

Schools are often forced to hire teachers who have more seniority and veteran teachers have been allowed to “bump” newer educators for the most desirable positions.

“These contract rules thwart any sustained attempt to significantly improve teacher quality – the single greatest school-based factor in increasing student achievement,” the study said.

The schools chancellor, Joel Klein, and the authors of the report said reforms made to seniority rules in the new contract negotiated with the United Federation of Teachers is moving city schools in the right direction.

“By providing schools with more authority to hire, and keep, the right teachers for the right jobs, we have taken an important step toward a more equitable and effective system,” Mr. Klein said in a written statement.

The new contract gives principals more authority to interview and hire teachers, and puts an end to “bumping.”

The study looked at five major urban districts including New York City and San Diego. In the past year, only four of about 70,000 tenured teachers in those cities were terminated for poor performance.

The report drew the ire of teachers’ unions.

“It is shocking that a group that purports to speak for new teachers would put out such an anti-teacher report,” the president of UFT, Randi Weingarten, said. “Taken out of its protective clothing, this is simply a report that shills for management’s demands, not for a new teacher’s needs.”