Advisor - No lessons for certain period

Sometimes it may happen that the class simply doesn't have a lesson that can be on certain positions.

For example this school has forbid all subjects to be on 4th period:

This is however a problem in the default settings where a student cannot have a gap in his/her timetable.

Either you have to allow some subjects or you need to block this period also in the timeoff of the class. Then the software will know there is no period at that slot:

Of course this was simple example - it may happen then only certain teachers combined with subjects will result is some period where no lesson can be placed because either teacher or subjects cannot go there.