This question presumes that "smart, A - A+" students actually are a threat to anyone. First, "smart" and "A-A+" are not the same thing. Plenty of smart students earn average grates and even more A students are not smart. It is a matter of achievement - there is underachievment, overachievement, nonachievement, and achievement, and any one person can exhibit any of these behaviors at any time.
The threat you describe is a perception that says far more about the person feeling threatened than the person evoking that emotion. Any learner who feels threatened by other students would be well advised to get a grip and look within to find the source of their insecurity.
Birth is a kind of genetic lottery. There are a lot of ways to measure "smart" and achievement is more about motivation than smarts. There are 2 ways to look at achievement. One either runs a race because they love running and would do it even if there was no one else in the contest, or they feel compelled to outrun everyone else in order to avoid losing. The outcome looks the same, and you can't always distinguish between a "winner" who enjoys learning from a "winner" who is afraid of failure.
Good students are a threat to no one with a single exception. Measuring grades on a "curve" has value. But a normal curve is a measurement of 1) the effectiveness of a teacher's instruction, and 2) the validity of the test instrument. I never manipulated grades to match a curve, thereby creating competition among students for a very few top grades. But on those few occasions as a rookie when the bell of the curve fell either too far to the left or right of "average," I looked to myself for the answer. If I hadn't taught effectively, the curve would show it and I threw those grades out, re-taught the concept, and re-tested. The curve would also showed if a test (I always wrote my own) was too easy or too hard, so I sometimes threw those grades out, too, wrote a better one that was a fair measure of mastery.
If an unskilled teacher misuses the grading curve to limit the number of high and low grades, that creates a toxic environment in which only the lucky inheritors of good brains and the neurotic overachievers will definitely hog the goods, and they do become a threat. But it is not about the kids, it's about their teacher.