Fiction by Lois Duncan.
This book starts out with a group of high school students who get on the bus after school, not concerned that the man at the wheel is not their regular driver. Instead he is a kidnapper! (I don't consider revealing that fact to be a spoiler, by the way, seeing as the name of the book is "Ransom".)
This was an exciting story. I've recently been enjoying rediscovering Lois Duncan's young-adult books, many of which I read over twenty-five years ago as a teenager myself. They are definitely worth the re-read for me, and should be still quite readable for teens today.
I also read by this author: They Never Came Home