Go to "Schedules" -> "Game Request" (only available to schools and assignors).
The interface for schools is the same as the assignor's interface (see the related article). The only difference is that the games requested by schools do not go directly to the schedule but into a requested games list where the assignor can choose to put them on the schedule or can delete the request.
To enable this feature, go to:
Check "Enable Schools to Request New Games" and save.