As a teacher for secondary school, you don't need to go deep in the technical.

The best guy around to talk about bitcoin and blockchain, in my opinion, is andreas antonopoulos.

He is a very good speaker and writer, he can explains how this works very well, in a way anyone can understand

Check his youtube channel

https://www.youtube.com/channel/UCJWCJCWOxBYSi5DhCieLOLQ

Watch his speeches, and tell what you heard to the kids. Some of them will really like it.