Hi @dan ,
I am also a high school student working with TensorFlow. I personally started off learning Machine Learning from Andrew Ng’s courses and strongly think they could build the basics for students. To learn specifically about TensorFlow I used and would recommend:

TF in Practice (now called TensorFlow Developer)
TF: Data and Deployment

by @Laurence_Moroney . Apart from this, I think the official TensorFlow Guide is a quite systematic approach to learn. Being in High school myself, another thing that I think has helped me quite a lot to learn is OSS, maybe your students could make small contributions to TensorFlow while learning it; I started off with something as simple as fixing an error in a TensorFlow example!
PS: In case your students want to validate their skills they could also take up the TensorFlow Developer certificate exam; TensorFlow also provides stipends for students giving the exam (which is also how I gave the exam)!