How to specify that class can have max one 7th lesson or max one 0lesson?

Simple solution:
First define the quesion marked positions:

Then you can say the class shall have max one lesson on the question marked positions. This can be done in Class's details:

Complex solution:
The simple solution cannot be used if you want to specify two conditions. For example the class has to have max one zero lesson and max one 7th lesson.

In this case you can use advanced card relationship. Here you can specify the area and maximim number of the cards in this area per week.

Please check this picture:

We have specified that all classes can have max 2 lessons on the marked positions per week. So it will not happen that one class will have 3 zero lessons.

Notes:
- you can use the same cardrelationship for teachers, just change 'Apply to' combobox.