From an anonymous source on boston.craigslist.org...

"In Brookline, Washington Square, at the Driscoll School on Washington Street if I had to guess I'd say 725 Washington Street. It's 1 block East from the Washington Square T stop.

Its nothing fancy (its a school). Painted onto cement. Clearly it is not available during the school day. The area is accessible to the public the rest of the time, but I believe they don't like people there after dusk.

That's all i know about it."

From the satelite photo it seems like there is a large and small series of courts.