How can I define lesson without any teacher?

In case you need to create a special lesson without any particular teacher (e.g "lunch" or "recess"), or you have not decided about the teacher yet, you can do so in "Lesson dialog" and select the last general option in the teacher's list - "Without teacher".

Note:
These lessons will not be visible in the "teachers view"

See also:
What do the colors in lesson grid mean?