Upon receipt of the minor-in-need-of-care petition, the children's court shall set a date for the hearing which shall not be more than 15 days after the children's court receives the petition from the presenting officer. If the adjudicatory hearing is not held within 15 days after the filing of the petition, it shall be dismissed unless;
(a) The hearing is continued upon motion of the minor; or
(b) The hearing is continued upon motion of the presenting officer by reason of the unavailability of material evidence or witnesses and the children's court finds the presenting officer has exercised due diligence to obtain the material evidence or witnesses and reasonable grounds exist to believe that the material evidence or witnesses will become available.