All applicants apply online, using either the Middlesex application through Gateway to Prep Schools (Gateway) or the Standard Application Online (SAO). Welcome to Middlesex Borough School District!To establish a complete pre-employment file, please complete the online application. The steps for applying for your substitute teaching certification are described below. 1. A substitute credential applies to a person who temporarily replaces a certified and regularly employed classroom teacher who is absent. Application Process. Fill out our online survey to indicate your interest in this program.