Anna,
We're are currently in the testing phase of supplying changes to the booking system.  We're going to set time limits on how far in advance students can and cannot reserve lessons.  So that should help you make sure you have enough time to see the lessons request.

We do not plan to include a feature which blocks students from booking at a time you're not available (according to your time schedule).  We encourage all students to take that into account when suggesting a time.  If you can't make the scheduled lesson time, we suggest that you modify the time and send it back to the student for approval.  Otherwise you can just reject the session.

and another thing...
I just had a request for a trial lesson at a time when I am not available. Is it possible to stop lessons being arranged at a time which is not on my timetable?

I recently missed a lesson because I didn't respond in time. The student chose the time and I simply hadn't read the email before the lesson was due to start. It would be better if the teacher could accept, or decline, before any arrangements are made.

italki is developing a system to deal with No-Shows (both teacher and student) that will hopefully be fair to both parties.  We are planning to display Absences on both teacher's and member's profile; this will affect the person's rating within italki.  We are also developing a way to compensate the teacher for No-Shows.  In the meantime, your session should have entered a 'dispute,' which we will try to resolve in the most fair manner possible.

If anyone has any suggestions on how to deal with these issues, we welcome your thoughts:
Student No-Shows
Teacher No-Shows
Teacher Repeatedly Doesn't Respond to Session Request
Student No-Show on Free Trial (Free Trial system coming soon...)