And while attendance was robust at Jackson, it appeared uneven throughout the district. Teachers at other schools reported near-empty classrooms all week.
At Roxborough High, for instance, some classes have only two or three students. A program for incoming ninth graders had fewer than 10 students.
"It's a really nice thing to have, but in a year like this, spending this much money on summer school is unconscionable," said one summer school teacher, who asked not to be identified for fear of reprisal.
A teacher at Barry Elementary in West Philadelphia had no students show up for her class. In most classes at Barry, fewer than 10 showed up, the teacher said.
"I just sit there," said the teacher, who also asked to remain anonymous. "There's nothing to do."
One class at a North Philadelphia elementary school designated a Promise Academy - an overhauled school designed by Superintendent Arlene C. Ackerman to mandate summer instruction for all students - had two students and three teachers. Attendance schoolwide was light, as it was for Saturday classes at the school, teachers said.
"I like the idea of giving the kids more instructional time," one teacher at the school said, "but if they're not there, it's a waste of money. How many programs that help kids all school year could you keep if you canceled summer school?"
District spokesman Fernando Gallard said the district had planned on 28,000 students but was calculating a more exact attendance. Based on that number, teachers will be reassigned Monday - and some could be dismissed if enrollment is too low, he added.
Most of the $18 million pays for teacher salaries for the 18 days.
Under Ackerman, the district has bulked up its summer school, offering a morning session of reading and math for those who need to make up work or want to advance, and an afternoon session of enrichment activities.
The district has cut summer school spending from last year's $40 million.
Officials defended summer school as a strong program that has helped students improve on state test scores.
"We believe that a high-quality summer learning experience has the power to yield benefits long after the summer is gone, making a real and lasting difference in the lives of children and families," Deputy Superintendent Leroy Nunery said in a statement. "Summer school is a proven investment."
An internal report evaluating last year's summer school found that students who attended 16 days or more showed "significantly greater gains in reading and math achievement than peers who did not participate in summer programming."
Councilman Bill Green, who has criticized the district's fiscal priorities, said summer school should be targeted only to those students who need to make up credits to move to the next grade or graduate.
"The School District is spending too much on summer school," he said.
During recent budget deliberations, Green said, City Council initially asked for the district to cut $10 million from its summer school budget, but the district returned with a $5 million cut. Now, the district has to find more savings to make up for a state funding shortfall of $35 million.
"The fundamental, overarching point is that it is not about whether something works or doesn't work when you have to deal with this kind of cut," Green said. "It is about what is more efficacious or less efficacious, and they didn't prove to me that this program is more important than other things they're going to cut."
Parent Helen Matthews said that she understood the budget pressures, but that she believed summer school was an important program. Her son Max is autistic, and summer class isn't a luxury for him - it's a necessity.
She understands some parents' criticism of some of the district's spending priorities, but summer school "is not on the list of bad programs," she said. "Some kids really need it."
This year is the second summer of SLAM for Max, 7.
"He learned a lot last year. It really helped him succeed," said Matthews. "I feel really strongly that this program is important."
On a recent day, Domb poked her head into a room of 22 eighth graders at Jackson.
Some were working on computers with one teacher, and others - students new to America - were in a circle, practicing reading and speaking English with another teacher.
"For them, 18 days of English instruction is huge," Domb said. "They'll be less afraid to speak in front of their peers."
The Jackson site had more than 300 attend last week, taking in students from Jackson, Meredith, McCall, Kirkbride, and Abigail Vare Schools.
One Jackson summer school teacher, who also asked to remain anonymous, said students who attended summer school would definitely be more ready to learn in September.
"But it's a huge expense, and most of these kids are not here for completion," the teacher said. "They're here because their parents need something for them to do while they work. It's a valuable program, but it's not necessary to spend this much."
Contact staff writer Kristen Graham at 215-854-5146, firstname.lastname@example.org, or @newskag on Twitter. Read her blog, "Philly School Files," at www.philly.com/schoolfiles