Sec. 16. "Strike" means:
(1) concerted failure to report for duty;
(2) willful absence from one's position;
(3) stoppage of work; or
(4) abstinence in whole or in part from the full, faithful, and proper performance of the duties of employment;
without the lawful approval of the school employer or in any concerted manner interfering with the operation of the school employer for any purpose.