When I was in high school we used BASIC and then Turing to learn basic instructions. With Turing we were able to learn to draw some basic graphics to the screen, etc. I'm not sure how applicable it would be to web app cases, but it was at least a little engaging at the time.
http://tristan.hume.ca/openturing/
To build a full web app for the first time without previous exposure could get messy quickly given the current landscape. Maybe start with a simpler website that includes laying out some HTML, styling it, and adding some basic front-end interactivity.
If there's a pre existing server that they could interact with, maybe that would help with encouraging them by giving them more immediate results without skimming too much of the detail into what they're actually building, rather than just learning too much by rote.
A bit of a search pointed me to Udemy which has a free course covering HTML/CSS laid out already. It wouldn't be too hard to build on something like that to show how Javascript interacts with elements on a web page.
https://www.udemy.com/build-your-first-website-in-1-week/