I do happen to be a science fiction person myself. Alas, there are not very many novellas published these days It's almost all VERY long novels or series.
I can think of any number of s-f novels published in the 1950s that would be short enough, but they might not appeal to kids today. A personal favorite is "The End of Eternity" by Isaac Asimov. I recently reread it and loved it once again. But I had also first read it not long after it came out.
Two novels by James Van Pelt come to mind: "Summer of the Apocalypse" and "Pandora's Gun". Check them out. They're considered YA and so would read quickly. Jim taught high school English for a very long time, and is only recently retired. So he's very understanding of h.s. age kids. He's mostly a short story writer, and many of his stories take place at school.
I will tell you this: s-f writers are incredibly approachable. I attend various cons each year (and as I type this I'm in Albuquerque for Bubonicon (https://www.bubonicon.com/activities/participants/) and over the years I've gotten to know any number of wonderful writers.