I worked as a teacher for 51talk. With students able to book lessons 1/2 hour before they commence with NO NOTIFICATION sent to teachers it is a totally unworkable situation. And top it off with US$16 penalties if 1 second late for a lesson which only gives you US$7.50 you can continually owe more money than you make. Everything is setup for 51talk to extract fees and penalties from teachers.
Wake up 51talk and create notifications for teachers... It is ridiculous the systems you have in place for any teacher to plan their day and have productivity.