Mich. Comp. Laws § 257.1827

Sec. 27. (1) A school bus shall be equipped with safety glass wherever glass is used in doors, windows, and windshields. Rigid safety plastic which meets the test requirements of American national standards institute standard Z26.1-1966 may be used on school buses in lieu of safety glass, except that front windshields shall be equipped with safety glass. School buses shall be fitted with at least 1 push-out window sash on each side of any school bus on which the plastic panes are installed. (2) As used in this section, "safety glass" means a product composed of glass, so manufactured, fabricated, or treated as substantially to prevent shattering and flying of the glass when struck or broken.