About what are the parents in the bus protesting?
So, you have four words to convey the notion that parents are protesting about buses. BUS PROTEST BY PARENTS would do it, but that's too passive. Instead, you go for PARENTS IN BUS PROTEST. That's putting the parents first, not the bus protest. Unfortunately, as it doesn't have a verb, it's not especially active either. It's also ambiguous, because you can read it as if it does have a verb — PROTEST — which renders it passive again. You could have removed the ambiguity by going with PARENTS' BUS PROTEST but you're worried that your readers' understanding of the use of apostrophes may be stressed by that.
Maybe you should just use more words? It worked for DEALER SHUNS THE TOILET FOR 23 DAYS TO HIDE DRUG WRAPS on page 16.