Except as otherwise provided in this code, no child under the age of fifteen years shall be employed for any purpose by any person, company or corporation, in this state during the hours which the public schools of the district in which such child resides are in session, unless the said child shall present a certificate from a school superintendent as provided for in RCW 28A.225.010, excusing the said child from attendance in the public schools and setting forth the reason for such excuse, the residence and age of the child, and the time for which such excuse is given. Every owner, superintendent, or overseer of any establishment, company or corporation shall keep such certificate on file so long as such child is employed by him or her. The form of said certificate shall be furnished by the superintendent of public instruction. Proof that any child under fifteen years of age is employed during any part of the period in which public schools of the district are in session, shall be deemed prima facie evidence of a violation of this section.