A board of education may establish policies for granting leave of absence including sabbatical leave, maternity leave, sick leave, and military leave. The board of education of a school district may, upon the written request of a teacher, and for good cause shown, grant a leave of absence or place him on a part-time teaching schedule for a period of one year, subject to renewal from year to year. Leaves and military service shall not be counted as continuous full-time service in computing tenure but shall not impair the tenure previously acquired by teacher under sections 168.102 to 168.130 nor affect any credit toward tenure previously earned. Any teacher under sections 168.102 to 168.130 who is called into active military service with the Armed Forces of the United States is eligible for reinstatement upon his discharge from said service without loss of tenure. Any permanent teacher employed on a part-time basis, whether at his request or not, shall retain permanent status. Any probationary teacher employed on a part-time basis, whether at his request or not, shall retain all credit earned toward permanent status and shall continue to earn credit on a pro rata basis toward permanent status.