Lemmings are notorious for following the crowd, usually in the form of flinging themselves off a cliff. So, the best answer here is your third choice, snickering about Colin behind his back. Since it's saying that everyone is doing it, this implies that everyone is following the crowd by doing it.
The other answer choices don't make much sense. Knowing a lot about the Magna Carta or even everyone raising their hands or staring at their shoes doesn't imply any type of herd mentality. It implies that people are familiar with the subject or don't want to seem like they know the answer. All of these are normal reactions when in a classroom setting and even in a business setting.
