Throughout the country, brutal budget tightening has reduced summer-school offerings in some districts and eliminated the classes altogether in others.

But in Philadelphia, 39,000 public-school pupils - almost double the number who attended last year - will start a $30 million, new-look, no-sweat summer school tomorrow.

Beyond basic morning classes for struggling students, the afternoons will be filled with enrichment sessions such as field trips, dance classes, technology camp, and environmental education, open to any child in kindergarten through eighth grade. And, for the first time, students won't be sweltering in rooms without air-conditioning.

Funding for summer school comes from the $126 million set aside for the first year of "Imagine 2014," Superintendent Arlene Ackerman's five-year strategic plan. For the first time since 2006, the district has a balanced budget and money to invest in new programs, thanks in large measure to federal stimulus funds.

Readying 132 sites for the 22-day program has been a sprint.

On Ackerman's directive, more than 430 window air-conditioning units are being installed throughout the district, said Fernando Gallard, a district spokesman.

Ackerman "decided that if we were going to ask our students to be in school, and entice them to come to school for a longer day, that we needed to provide them with a comfortable classroom," he said. "The same goes for our staff, who will be spending longer hours in the classroom."

In some older buildings, wiring and insufficient electrical current could present obstacles. "If somehow, somewhere, we're unable to air-condition a room, there will be sufficient fans to keep students and staff cool," Gallard said.

Morning classes are being offered for all students in kindergarten through eighth grade in the district's empowerment, or most struggling, schools. The sessions also are open to all K-8 students who are not being promoted, those with disabilities, and English-language learners.

High school students who have failed two or more core classes can gain up to two credits.

Afternoon enrichment courses, offered Tuesdays through Thursdays, are open to all K-8 students in the district. And high school students will have opportunities to enroll in art and instrumental music camps, as well as other programs.

Although summer school's duration - 22 days, ending July 31 - has been criticized as too short to be meaningful, the district has imposed new standards this year. Those attending summer school for promotion or credit will have to maintain a 70 average based on a midterm and a final and miss no more than three days. There will be new weekly assessments as well.

When the new summer school was announced, officials predicted that as many as 45,000 students might attend. As of Friday, that number had reached 39,000 - 31,000 in morning academics, 8,000 in afternoon enrichment.

Gallard said that the district had secured 1,600 teachers, enough to educate the summer population. More teachers began signing up when Ackerman offered a $1,000 bonus to those who agreed to teach in struggling schools.

Also new this year is the Summer Bridge program for ninth graders entering seven comprehensive high schools: Bartram, Fitzsimons, Germantown, Lincoln, Overbrook, University City, and West Philadelphia.

The schools, all of which received sizable grants from the Department of Labor to enhance education and career opportunities, will offer programs to prepare about 1,000 incoming freshmen for high school.

Studies indicate that students who fail to complete ninth grade on time are much more likely to drop out than those who pass their freshman year the first time.

"One of our biggest challenges is trying to get children to 10th grade on time," said Courtney Collins-Shapiro, director of the district's Multiple Pathways to Graduation office. "We know that where we're losing most of the kids in these schools is in ninth grade."

Summer Bridge classes will not be dry, teacher-in-front-of-the-classroom experiences, but will incorporate online learning and include trips to colleges, technical schools, and museums. Five weeks of "intense" four-hour days will make students aware of the resources available to them in the high schools, Collins-Shapiro said.

This fall, they will continue in a mentoring program to last through 10th grade.

It fits in well, Collins-Shapiro said, with the district's transformed summer school.

"We're trying to get the message out that this isn't traditional summer school," she said, "that this is a mix of academics and fun stuff."

Contact staff writer Kristen Graham at 215-854-5146 or kgraham@phillynews.com.