Sustainable building developer Project Frog has raised $4 million in the first tranche of a second round of funding led by Rockport Capital Partners. The total goal for the round is $8.6 million, which will go toward supplying schools with materials for energy-efficient classroom modulars.

The San Francisco-based company, a spinoff of environmental architecture firm MKThink, charges about $150,000 to $200,000 per classroom, and offers schools the option of installing solar panels to save even more energy, reports Alarm:Clock. But the demand is there, with a climbing number of college and high school students attending class in portable trailers (about 3 million currently).

While details on the initiative’s first round of funding are undisclosed, Rockport has a long track record of funding other sustainable projects. The firm just led a $37 million round for Wind Power Holdings. It also partnered with Shasta Ventures to deliver $6 million in first-round funding to Sustainable Spaces, another S.F.-headquartered building company, and again very recently with GE Energy Financial Services to fund photovoltaic manufacturer Soliant Energy.