Two 16-year-olds have been charged with attempted murder.
Walter has left Overbrook and is completing his senior year in an undisclosed location.
"We were all just stunned," Overbrook principal Ethelyn Payne Young said.
Guidance counselor Lorraine Battle, who last spoke with Walter by phone about two weeks ago, said he was on track for graduation, had been accepted by six colleges, and was staying upbeat despite the brutal setback.
"His spirits are good," she said.
School district officials said they were working with Walter's new school so that he could receive a diploma on time and get to college.
"He's a good kid, and we want to make sure good kids get the rewards they're supposed to get out of this world," said Michael Silverman, superintendent of the city's neighborhood high schools.
Such violence often stalks city students trying to overcome poverty, crime, and absent parents through education.
Philadelphia Futures, which mentors promising teens through college, saw one of its students, Tyrone Myers Jr., murdered in a neighborhood robbery in 2006, just shy of a degree from Pennsylvania State University. Another recently was mugged on his way home from a book-club meeting.
"How do you cope? I guess it's part of the territory, part of their lives," said executive director Joan Mazzotti. "Because their needs are so deep, our services are so intensive."
It is not clear from court records what prompted the attack on Walter. The District Attorney's Office declined to comment.
None of the youths attended Overbrook with him, though he told police he knew some of them.
Walter did not respond to The Inquirer's calls and e-mails, but in an interview last summer, the drum-line member with a flair for writing reflected on his determination to achieve.