I need help in designing (zoho style) an application that will enable an organization to organize it's calsses. 

The appliaction will have : Students, Tutors and Classes (physical classes) that needs to be pre booked. 

the main question I have is how do I make the logic that an instructor can NOT be in 2 classes at the same time, etc... (I don't need/want to lay to you all the criterias, as I don't think it matters).

i have a problem in the design (if possible) and also on an easy "Make a new lesson"  button (something like haveing a weekly/daily view of a calendar, and selecting start and end hours.

i thought of doing it with drop down menu, but I have problem marking the free time slots.

thanks