You can easily define the free times of the teachers. When you click on the button "Teacher Freetime & Courses" you can enter on which times the teacher must not be scheduled.
To enter the free times you just click into the left column of the left table in the row with the asterisk. A combobox will appear where you can select two values:
If you click into the "Days" column you can select for which day you want to set the free time period. You can either select "ALL" or the specific day.
The next columns are for the desired semester and the lesson(s) which have to be free time.