Subjects time-off
In cases, when the subject can be taught only on specific days or periods, you can use the "Time-off" setting and allow or block positions for all lessons of this subject.
Use menu Main and click on Subjects, select the subject and click "Time-off". 
In the new dialog you can change the state of each cell by clicking on it to "Available", "Conditional" a "Not available". 
If you click on rows or columns headers, you can change the state for all cells in the row/column. 
E.g. - if all swimming lessons can be only on Monday, Wednesday and Friday afternoon you can change "time-off" setting for subject "Swimming": 
Or, if you need to lock all lessons from one subject on first period each day (e.g. "Morning circle"), use this setting:  
Note:
- in case, you need to block subject on positions only for some classes/teachers, use: Biology has to be in the afternoon in certain classes and in the morning in other classes
See also:
Max one history lesson on 6th-7th position in each class (solution 1)
 
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