Today, the Los Angeles Unified School District has a goal of converting at least 30 percent of every schoolyard to green space, a years-long project that it expects to cost $3 billion. By its own estimate, about 475 schools do not meet that standard and, of them, more than 200 elementary schools have less than 10 percent green space. This analysis does not include school parking lots or truck delivery areas — paved surfaces that are likely to remain that way and raise the temperature around schools.

Webster, after years of waiting, is now on the list of schools to be renovated by the Trust for Public Land. The nonprofit will work with a class of third-graders and landscape architects for the next year to design a new schoolyard. Projects like this can take two to three years to complete, at a cost ranging from $400,000 to as much as $2.5 million, said Danielle Denk, who directs the organization’s schoolyard transformation work. In Philadelphia, most of the money for these projects comes from the water department, which is trying to make the city more capable of absorbing storm runoff.