Karyn Lynch, deputy chief of student services, emphasized that people trained in CPR and first aid responded to the child as soon as they became aware he was in distress.
"The minute they looked over and saw him, they immediately performed CPR and called 911," Lynch said.
It was not clear whether the child had a preexisting medical condition. School officials cannot discuss students' medical histories.
Asked whether having a nurse in the building would have made a difference in the child's death, Gallard said: "That's a very difficult question to answer, not knowing what occurred. We're not medical experts. There were trained personnel with CPR certification."
And, Gallard said, "he was transported to one of the best hospitals in the nation."
The school is in shock, Lynch said. The boy had a sibling at Jackson, and his classmates witnessed his distress, then saw him being taken away to the hospital.
On Thursday, there will be supportive services at Jackson - psychologists and bereavement specialists on hand.
"They'll be there for the school day, and for as long as they're needed," Lynch said.
The boy died late in the school day, Lynch said, so some students may not learn about it until they arrive for classes Thursday.
It was the second death involving a Philadelphia student this school year. A sixth grader at Bryant Elementary died after suffering an asthma attack during the school day. There was no nurse at Bryant when Laporshia Massey became ill.
Helen Gym, a founder of Parents United for Public Education, was stunned by the latest death, which she called an unspeakable tragedy.
"It's a fundamental responsibility of the schools to provide for nursing care," Gym said. "You cannot take these reckless ideas that somehow you can slash essential people and personnel and staff at schools and not think that consequences won't happen, that tragedies won't happen."
Teachers union president Jerry Jordan said he did not know the circumstances surrounding the child's death, but said, "I do know that the building is woefully under-resourced. And now we have lost a baby. This is horrific."
Jordan noted that the district was contemplating more staff cuts if it does not get at least $216 million in extra money from the city and state for the 2014-15 school year.
"What are we going to do?" he asked. "Just keep screwing around until we allow more terrible things to happen to children?"