Mich. Comp. Laws § 388.553
No person shall teach or give instruction in any of the regular or elementary grade studies in any private, denominational or parochial school within this state who does not hold a certificate such as would qualify him or her to teach in like grades of the public schools of the state: Provided, however, That any person who shall have taught in any elementary school or schools of the standard specified in this act for a period of 10 years or more preceding the passage of this act, shall, upon filing proof of service with the superintendent of public instruction, be entitled to a certificate by said superintendent of public instruction in such form as he shall prescribe, to teach in any of the said schools within the state: Provided further, That teaching in such schools shall be equivalent to teaching in the public schools for all purposes in obtaining a certificate: Provided further, That the teachers affected by this act may take any examination as now provided by law and that the superintendent of public instruction may direct such other examinations at such time and place as he may see fit. In all such examinations 2 sets of questions shall be prepared in subjects ordinarily written on Saturday, 1 of which sets shall be available for use on Wednesday by applicants who observe Saturday as their Sabbath: Provided further, That any certificate issued under or by virtue of this act shall be valid in any county in this state for the purpose of teaching in the schools operated under this act: Provided further, That any person holding a certificate issued by the authorities of any recognized or accredited normal school, college or university of this or other state shall be entitled to certification as now provided by law: Provided, however, That teachers employed in such private, denominational or parochial schools when this act takes effect shall have until September first, 1925, to obtain a legal certificate as herein provided.
MCL 388.553