The CompTIA Network+ book 8th edition (2022). These books aren't just for preparing for the CompTIA exams (that's a side benefit if you decide you want to get a cert later) but really are great to read on their own and learn everything you didn't know you needed while being curated. Good for those students who don't have a CS degree like you explained why the other books aren't suitable. Your local public library will usually have one on the shelf, check it out! Select the chapters you want to teach and skip the others. If you want something even more introductory, you can look at the CompTIA A+ book 11th edition (2023), skip the IT specific and focus on just Chapters 18-21 (Chapter 18 Essentials of Networking, Ch 19 Local Area Networking, Ch 20 Wireless Networking, Ch 21 The Internet) as reference for something even more basic but still well structured. This book is especially tailored for those just starting out and more than suitable for a highschool audience.