PHP Tutoring Application

I am looking to see if there is already an application built for booking tutors online. Before I write one just looking around to see it one exist. I have found "todo calendars applications" and "meeting room booking applications" on www.sourceforge.org, but no luck with tutors booking online. I am not looking for a complex one, students log in and select a topic and pick a day and time for the help lesson. The database will have a list of available instructors/teachers that the student's help lesson will be linked to. Of course it will also have to know when no instructors are available because they are all booked or it is a holiday If anyone knows of an application like this out their can you let me know. I don't want to write my own if one already exist out there. Thanks, Mark.