Use a text editor such as notepad to edit the "simulation.ini" file in the "c:\program files\cat daddy games\school tycoon\data" folder. Make the following changes:

[COST_VARS]
BaseTeacherSsalary = 0
TeacherSalaryPerSkillPoint = 0

Note: The value for "TeacherSalaryPerSkillPoint" does not have to be 0, but 0 makes teachers free.