Riverfront complex, media center, telescope. These are Philadelphia public schools?

On the second floor of an under-construction school building, John Swoyer stepped onto a deck overlooking the Delaware River and a swath of freshly seeded land covered in straw.

"I just envision this place filled with kids," Swoyer said, lifting his phone to take a photo of the future sports fields between I-95 and the river in Tacony. Rather than the Philadelphia School District, the new school will be used by students at multiple campuses of the Mathematics, Sciences and Technology Community Charter School — known as MaST.

The nationally recognized Northeast Philadelphia charter school, which opened in 1999 and this year claimed a waiting list of more than 11,000, is growing. MaST II opened in 2016 in rented space in a former Catholic school in Lawncrest; next year, students in two grades will be the first to attend school in MaST II's brand-new building in Tacony.

And a third MaST is also opening next year. School leaders are hoping to close on the Crown Holdings Inc. headquarters — which is "in pristine condition," said Swoyer, MaST's CEO. "It's a huge building."

While the Philadelphia School District has struggled to fix environmental problems amid $5 billion in repair needs, MaST, which like other charter schools is publicly funded, has been buying property and building with proceeds from bonds.

"The charter schools have assessed that it's worth it … to invest in their school buildings," said Mary Filardo, executive director of the 21st Century School Fund, a Washington-based nonprofit that advocates for improving urban public school facilities. So rather than using all their per-pupil funding for instructional purposes, such as teacher salaries — which tend to be lower for charters — they are using part of it to repay revenue bonds for their facilities, she said.

Growth has leveled off in recent years, but the charter school sector in Philadelphia has still doubled in size in the last decade — now enrolling about 70,000, or one-third, of public-school students in the city — and funding remains a contentious issue.

The School District sends money to charter schools based on how many students they enroll. For MaST — which is planning a second school building on the Tacony property in addition to the $20 million project still underway — how soon it will finish its campus depends on whether the school board will permit it to add enrollment.

School leaders are asking the Philadelphia school board to permit an additional 650 students at MaST II as part of its charter renewal this year, Swoyer said. Enrollment at the school is capped at 1,250.

"More students allows us to borrow money, grow quicker," Swoyer said.

If MaST buys the building it's eyeing for its third outpost, it will have space to expand there as well. The now-disbanded School Reform Commission in February approved MaST III to enroll up to 1,300 students — half of what school leaders had requested. Swoyer said the Crown building could hold 1,900 students.

According to Swoyer, the three MaST schools have received more than 16,000 applications so far through the new Apply Philly Charter process, which allows families to apply to multiple Philadelphia charter schools with one application.

"Our vision is to complete these campuses," Swoyer said. "We want to fully build out capacity to help kids."

MaST has been financing its expansion through bonds. The Philadelphia Authority for Industrial Development issued two series of revenue bonds in December 2016, for $28.6 million and $19.9 million.

Besides building the Tacony school, bond proceeds went toward buying the land for the school, remediating environmental contamination, and advance refunding of bonds issued in 2010. Proceeds also paid for an addition to the original MaST campus, including more space for art and music classes.

Upon entering the original MaST building — the former home to a steel storage facility — visitors are greeted by 16 flat-screen monitors mounted on a wall, displaying pictures of students, activities, and the new MaST campus.

The school's hallways are designed as "neighborhoods" for each component of its STREAM focus — science, technology, robotics, engineering, arts, math — with floors painted like roadways, and streetlights on the walls ("We got this on eBay," Swoyer said, pointing.)

The school boasts an array of technology, including a 3D printer that freshmen this fall used to design small mazes. During a visit to the school, high-school students were brainstorming games for a robot Olympics.

A media center features cameras and different types of lighting. On the roof sits a telescope that Swoyer described as the second-largest in Pennsylvania. Outside the building, a parking lot converted into a playground includes a track for racing robots.

Other features at the school include a "build-it" center with Lego-board tabletops, and a library with an artificial flame in a fireplace designed "for kids to feel like it's a college space," Swoyer said.

To supplement its funding, MaST holds an annual raffle — "We try to raise $90,000 to $120,000," Swoyer said — and uses the proceeds to make improvements to the school. The school has also received donations, including the 3D printer. The Philadelphia School Partnership, a nonprofit that awards grants to charter, parochial and district schools, provided $1.9 million for MaST II's startup.

The school also recently received a $900,000 grant from the U.S. Department of Education's Office of Innovation and Improvement for MaST II's expansion.

“We’re trying to build up a support pool. Now that we’re growing, I think more people are willing to look at that,” Swoyer said.

Correction: A previous version of this story misstated the enrollment cap for the MaST II charter school.