Substitute Teacher Service, Inc. | 2,520 followers on LinkedIn. Welcome to Substitute Teacher Service, Inc. Your go-to for top substitute teachers, aides, & support staff. | Substitute Teacher Service, Inc. (STS) proudly serves as Pennsylvania's leading provider of substitute teachers, teacher aides, and support personnel. Our primary goal is to deliver exceptional customer service while providing cost-effective solutions tailored to meet the diverse needs of our client school districts.