All K-12 schools in the United States have access to a free ArcGIS organizational account, this includes Public, private, and homeschools. There is even a Club Kit that has fewer licenses to be used for clubs or homeschools. You can see if your school has a license here.
If your school is not on the map, it is easy to sign up for an account by filling out the form on the bottom of the Schools Mapping Software Bundle for Grades K-12 website. Without an account you can still access most of the GeoInquiries and some story maps but if you want your students to be able to create their own content and use analysis tools you must have an organizational account.
Esri has many tools for teachers to begin their journey in GIS. The Learn ArcGIS website has many tools and guided lessons to help you get started. Story map galley allows you to search pre-made story maps
This tool kit will include many resources to get you started as well as ideas to help you expand your skills and knowledge. It is broken down by subject area, however many resources can be used across disciplines.