How can I define lesson without class? Eg. Teacher's meeting or preparation time

without class, teachers' meeting,

Lessons without class can be used to specify some activities for teachers only. These lessons can cover some administration work, teachers' meetings, preparation time, etc...
For creating them, just select the teacher (or more teachers via "More teachers" button), subject and as a "class" use option "without class".

A good example is to create a lesson with all math teachers without any class. By doing this you can make sure that all math teachers have time to meet and discuss some math teaching strategies at the same time every week.

Maximum one planning time lesson per day