Eileen Tutaj, 47, in a phone call from California, said an autopsy will determine what felled her "wonderful boy," Joseph.
Thursday night, she was arranging to fly home to Long Island without her 15-year-old son's body and await the results.
The mother, a physician assistant, said her son was healthy when he left on Valentine's Day for a weeklong trip with the high school's marching band that included performances and tours to Disneyland.
"This was supposed to be such a great trip for him," she said. "He couldn't wait to go."
School district officials would not agree to be interviewed Thursday. In a statement, Seaford High principal Scott Bersin said Joseph was "widely known and respected for being a kind person."
Joseph Tutaj began feeling ill Saturday and was in and out of local clinics and hospitals for the next several days, his mother said, but never was admitted overnight. He kept in touch via text messages, she said, and spoke of running a high fever.
On Tuesday evening, Eileen Tutaj was at Kennedy Airport, trying to get a flight to be with her son, when a doctor called to tell her that Joseph had died.
The autopsy will be conducted Friday or Saturday, said Ed Winter, a spokesman for the Los Angeles County Department of Coroner. He said Joseph Tutaj was "with friends and collapsed," according to a report that was sent to his office.
The sophomore was among about 80 band members who traveled to Los Angeles to perform and visit area attractions.
Described by a school friend as quiet but friendly, Joseph started playing the French horn when he was in the fourth grade and joined the band in high school, his mother said.
By then, he had moved on to another instrument -- the trumpet -- after discovering his great-uncle's instrument in a closet.
He had been planning for the band trip for months and kept in close contact with his mother through text messages, she said.
Eileen Tutaj said her son told his chaperones late Saturday afternoon that he was feeling ill, with a cough and chills. They took him to the Disneyland medical treatment station and he returned to the hotel later that day, his mother said.
On Sunday, Joseph went to a clinic and then was taken to a hospital. His fever spiked to 105 that day, his mother said, but quickly came down.
He sought no medical treatment Monday, she said, but was feeling sick again Tuesday before the flight home that night.
"On Tuesday, he wasn't feeling well and couldn't fly, so I made arrangements for my college friend to come get him," his mother said.
He was a member of Boy Scout Troop 581 and had attained Life Scout rank. The mother said his troop leader told her Thursday that he will try to earn her son the prestigious Eagle Scout designation posthumously.
A Seaford High School junior who was a troop member, speaking on the condition that his name not be used, said Joseph had a wide swath of friends.
On Wednesday, groups of students huddled together, crying, and Thursday the school was quiet and somber, he said. In their grief, "people who barely talked were connecting," he said.
Bersin, the principal, said the teenager was quick to volunteer -- he was a stage-crew member for a recent high school production -- and was a gifted computer programmer.
Eileen Tutaj said her son was a member of the Model Congress Club and had talked about attending college since he was a little boy.
His sister Emily, 12, is heartsick. "They were very good together, always looking out for one another," their mother said. "She is going to miss that." With John Hildebrand