Lessons for specific subjects can be over lunch free period

lunch, subjects constraints,

For each class you can define "lunch free periods" for a day - see: How to specify when a class can have a lunch?

However, if there are subjects, which you consider, that can be during this free periods (e.g some art courses), you can set it in Subjects constraints - Can be over lunch

Then, if there will be no other way, program will place such lesson in the timetable even if it would mean, that there will be no free period for the lunch in a day (and the software still considers the "lunch constraint" as fulfilled).