How to make the lessons of certain discipline be always last, for example, if this discipline is optional, and after this lesson there should be no more basic lessons on that day

Create such additional lesson relationship:

It means that if “Math. Optional” will be on 6-th position, after it no other lessons may take place on 7-th and 8-th positions.

Create similar relationship for the 7-th position (depends on number of lessons per day). I.e. if “Math. Optional” will be on 7-th position, then no lessons on 8-th position.

Another option is to choose that lesson must be last: