Places to play: Sacred Heart, Seisen, St. Mary's, A S I J

Each of these schools has 2 to 4 tennis courts.  Sacred Heart is in Hiroo, Seisen and St. Mary's on the far side of Setagaya ward, and A S I J is in Chofu (a midrange suburb of Tokyo).  Student usage takes priority, so Saturday mornings (or all day Saturdays) will often be blocked out for the students. 

Basically, to use these courts, you need either a) to know a parent who has a child at that school, and get that parent to invite you, or b) to be a parent yourself with your child at that school.

Each school has its own arrangements for non-student use of the courts.  Often those arrangements are posted at the courts, or you can call the administration or P.E. instructors to find out. A S I J has a sign-up sheet, while at St. Mary's people just seem to show up and take turns on the courts.