I am not a science fiction aficionado, but I absolutely loved the Orson Scott Card books Ender’s Game and Ender’s Shadow. They are written in a truly believable way, and he uses well developed human characters and intricate plots to take his readers along for a wild ride.
It depends of course how good your twelfth grader is at reading fluency, comprehension and literary maturity but I would say that an old classic adventure story involving daring deeds, the wilderness and the ever-popular animal subject would be a good start. I have in mind both Call of the Wild and White Fang both novels by Jack London. For example, in the first novel, the story is 'narrated' by a dog. This is a refreshing idea for young people, particularly those who may not yet have grasped anything sufficiently novel in their reading to 'hook them in' to a story. Buck is a dog who gets stolen by some ruthless men who notice his qualities of strength and stamina. They recognise in him the qualities that might help to make them rich in the gold rush up North. But Buck has other ideas....
Everyone has different tastes, of course, but I will share some of my favorite adventure/science fiction writers, and just a few of their books.
First are Douglas Preston and Lincoln Child, who write together and separately. Some of their books feature a very odd FBI agent who is always exploring some supernaturally-related crime. Here are a few titles: Relic, Cabinet of Curiosities, Still Life with Crows, and Dance of Death.
And for a good, old-fashioned adventure writer, there is H. Rider Haggard. His hero, Alan Quatermain, was featured in League of Gentlemen. His most famous book is probably She.
These are all books that are fine for a twelfth-grader, and I hope you enjoy at least a few of them. I would guess that all are available in paperback and at a public library.