In all school districts, including special charter districts and districts located in cities having a population exceeding 500,000, the local school board shall grant, for a period of up to 5 years, a leave of absence to those of its employees who accept employment with an alternative school, provided that the employee shall satisfy any leave of absence provisions that may exist under a collective bargaining agreement or, if such an agreement does not exist, a school board policy. At the end of the authorized leave of absence, the employee must return to the school district in a comparable position or resign. The contractual continued service status and retirement benefits of an employee of the district who is granted a leave of absence to accept employment with an alternative school shall not be affected by that leave of absence.