The first thing that I should mention is that PenPalSchools is made for teachers, by teachers. It hopes to make itself accessible to classrooms without financial support and so use of it is free; donations, however, are appreciated.
PenPalSchools is a Learning Management System (LMS) style tool. It allows participating teachers to manage the exchange from a central homepage. This is nice because there may be some concern about student participation, and even the maturity students may have when communicating with strangers from other parts of the world. Teachers can monitor exchanges and even delete messages if need be.