McCaffery had spent three months at the West Philadelphia school as a long-term substitute. On May 21, 2001, he said, he was struck in the head as he tried to separate two boys who had gotten into a fight. He suffered a concussion that required an overnight hospital stay.

Last week, McCaffery stiffly worked his way into a chair at a restaurant near his South Philadelphia home, nursing a back injury from a similar altercation Monday at another school.

He loves his students, the 54-year-old teacher said, and so many of the staff and administrators he works with. He estimated that he had worked in at least 35 Philadelphia schools in the last seven years, after a career in New York.

But he said legislation such as the No Child Left Behind Act had forced administrators to offer scripted - and potentially self-punishing - responses to violence. The threat of sanctions against a school or its personnel creates an incentive to ignore or downplay altercations, McCaffery said.

"It's important to speak out, for the interests of everyone - students, teachers, parents, administrators," he said. "Too often, when you're talking about school violence, the issue is swept under the rug."

Layered atop that are generations of children with poor transitional skills who struggle to move from home to school, class to class, task to task. The riskiest time of the day, McCaffery said, is when students are moving or about to move.

That was when he was injured both times.

The first time, at Heston, he taught a class that had gone two years without a permanent teacher, McCaffery said. It consisted of 15 third, fourth and fifth graders with serious academic and disciplinary problems. That may sound like young children, he said, but a fifth grader who starts his schooling late and gets held back a couple times can be a strong adolescent.

McCaffery doesn't know what sparked that fight. It was the last period of the day, and the students were restless.

Fernando Gallard, school district spokesman, said the district incident report noted that McCaffery had injured his neck.

On Monday, about 11:45 a.m. at Smith Academics Plus Elementary School in South Philadelphia, McCaffery was escorting a class on a flight of stairs when two fifth graders got into a fight. As he tried to keep them apart - and the other students moving, and his own balance - he wrenched his back.

McCaffery, his records show, was treated at Hahnemann University Hospital by Frank Burke, who diagnosed strains and sprains of the lumbar region. The physician prescribed a muscle relaxer commonly used to treat back injuries and scheduled a follow-up appointment.

McCaffery said he wasn't complaining. The Smith principal and staff did all they could to try to ease his pain and make sure he got treatment, he said.

Police were not called to the scene for either incident. The principals at the schools could not be reached for comment.

"There are wonderful people in the Philadelphia schools. I have a tremendous amount of respect for Paul Vallas," the district's chief executive officer, McCaffery said. "There are principals I work for who I cherish."

That helps explain why, even after twice being injured, he has never thought of finding another career, he said.

"I love teaching. I love kids," he said. "When I say I'm a teacher, that's a tremendously satisfying thing."