How to print only certain classes or teachers?

You can use button Filter. Here you can specify for each type of the objects allowed values that you want to see in the printouts.
For example, here we will print a summary timetable only for 3 teachers and only for Monday, periods 2-4:

This filter is global for all printouts, so it will affect all the printouts. For example if you select 3 teachers, then in the summary timetable of teachers it will print only 3 rows. In timetable for each teacher it will print 3 pages. However, printout for each class will not be affected one page for each class will be printed.
You can clear the filter using button “Clear filter – print ALL items” in the filter dialog.

Note: The filter is not saved, its reset after you load the timetable.