Elon Musk has been running a secretive, highly selective private school out of SpaceX’s headquarters in Hawthorne, California, Ars Technica reported.

The experimental non-profit school, Ad Astra, has been educating Musk’s sons along with children of SpaceX employees and a few others in the area since about 2014.

Musk originally founded Ad Astra to provide his five sons with schooling that exceeds "traditional school metrics on all relevant subject matter through unique project-based learning experiences,” according to a document filed with the Internal Revenue Service and originally uncovered by Ars Technica.

“I just didn’t see that the regular schools were doing the things that I thought should be done,” Musk told a Chinese TV station in a 2015 interview. “So I thought, well let’s see what we can do. Maybe creating a school will be better.”

According to Ars Technica, kids between the ages of seven and 14 work together in teams on topics of their choosing, with few formal assignments and no grading. Students can opt out of subjects they don't like. There is heavy emphasis on math, science, engineering and ethics — at the expense of instruction in languages, music and sports, Ars Technica reported. And everything from tuition to the Mac laptops used by children is funded by Musk.

Ars Technica reported that almost 400 families applied in 2017, though the school only has about 50 students and a dozen open spots at any given time.

It's too early to tell how effective the unconventional curriculum will prove to be. As his children age, Musk has continued adding grade levels to accommodate them, so no students have graduated yet from Ad Astra.

Read more about Musk's secret school from Ars Technica.