I think this might have been based on the Chowchilla schoolbus hijacking in the 1970s: a bus full of kids is kidnapped on a rural part of the route and held hostage, and the kidnappers get increasingly desperate when they realize they didn’t get the bus with kids from the rich part of town they meant to. I remember it being very scary and intense, and I was probably too young to be reading it (LOL). I would say I read it in the mid-1980s.
Lost Classics of Teen Lit: Name That Book! 
It was originally published in 1966 according to Google, but it could be Ransom by Lois Duncan, also published as Five Were Missing. I also read it in the early 80s and it really scared me.
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It could also be Under the Beetle’s Cellar by Mary Willis Walker
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I agree; sounds like the book Ransom. Lois Duncan was one of my favorite authors as a teen.
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Right… I think it IS one of the Lois Duncan books. Among the kids kidnapped were this awkward younger brother and his older brother who was the school hero. At the end, the younger brother causes some kind of a distraction, causing the kidnappers to drop the gun and then the older brother, Glen (I think?) picks it up and sort of is given credit for saving the day… which the younger kid thinks, “Of course, that’s how it ALWAYS happens.”
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