It is very important that any lessons which must be placed at certain times on the construction grid are placed in position and fixed before any other scheduling occurs. This is usually because a teacher, external to the school is only available at specific periods.
In the screenshot above, the Architecture lessons might be taught by a local architect and she is only available on Monday, Wednesday and Friday at 9.00.
Please note that if the lessons placed are part of a block, the other lesson in that block will also be placed in these gird positions when automatic scheduling takes place.
The video below will show you how this process is carried out.
Before you decide to fix lessons, please read the article An Alternative Method to 'Fixing' Lessons on the Grid