If you have received a 1099 MISC form for your substitute work, note that typically 1099 MISC forms are issued to workers that are considered "independent contractors" and that are not employees of the organization. In those cases, the IRS considers that income as self-employed income and you’re responsible for reporting your earnings and paying your taxes directly to the IRS.
Here is more information related to whether income is self-employment income:
https://ttlc.intuit.com/replies/4771022
https://ttlc.intuit.com/replies/4770969
Information on whether one is an employee or independent contractor:
https://ttlc.intuit.com/replies/4243263
