How to specify when a class can have a lunch?

If your students have the lunch during break between 5th and 6th period or if all your students can have their lunch after their lessons, you don't need to care about entering lunch in the program.

If all classes have lunch during 6th period, you can define this with Time-off for classes. Just check out period 6 in Time-off. (you can do this also if e.g. class 1-4 have lunch break on 6th periods and classes 5-8 have lunch break on 7th periods - just define Time-off for classes correspondingly). See Razredna pauza

However, if your students have to have the lunch during 5th or 6th period, and program has to choose one of these periods, you have to specify it, so that algorithm leaves some period empty for them each day. You can specify this in Class/Constraints:

If you for example specify that the lunch must be between 5-7th periods, the software will then leave one free period in that interval. Either 5th or 6th or 7th. So the students will have at least one period free for lunch every day.

You can also specify that you do not want to have teaching after the lunch in case they have lunch in the last period of the specified lunch interval.

A special case when you do not want to have just one lesson after the lunch:
Avoid single lesson after lunch break.