No person shall be employed to teach, as principal or assistant, in any school supported wholly or in part by public money unless the person shall have a certificate of qualification issued by or under the authority of the board of regents for elementary and secondary education. Provided, however, that any person who is employed as a part time speech and language pathologist at the Rhode Island School for the Deaf, for a period of not less than ten (10) years prior to July 11, 1990, shall not be required to have a certificate of qualification, and shall be exempt from the provisions of this section which require the certificate. In case any city or town shall pay or cause to be paid any of the public money to any person for teaching who did not, at the time of teaching, hold a certificate, then the department of elementary and secondary education shall deduct a sum equal to the amount so paid from the amount of the state’s money due, or which may thereafter become due, to the city or town, before giving his or her order in favor of the city or town for any of the public money under the provisions of §§ 16-1-10, 16-1-11, and 16-5-22.