I get asked a lot "What resources does Esri have for me to use or learn with?" The answer is "A LOT!" Rather than require you to go hunting, I'm presenting here a bunch from which you can easily choose as you see fit.
First, the good news. K12 schools can have software at the school at no cost for instructional use. Go to https://www.esri.com/schools, hit the pull-down, choose "Software Bundle" and just fill out and submit the form. Easy. (Administrative users and higher ed folks, see the "Licensing" tab at the far right on that page. International users, see your local Esri office.)
The harder part is making decisions about what to teach about and with.
A vast amount of Esri training is available for free, usable 24x7.
If you want to "teach about GIS" the way people teach CAD or video editing or programming, that's different. For CTE teachers, all the above may be useful, at least in terms of learning the technology and getting comfortable with why and how people use it in particular arenas. But for answering "What do we need to include in our course?", it's essential to know the requirements, what defines "success," and the experience of the instructors. I'll add two important sites for CTE leaders: