Gate to allow only one player to pass through

On my server I have a quiz which you need to take before you can go to most of the areas, build etc... However, sometimes people try to cheat by going with friends into there. I want some kind of gate/mechanism which only allows one player to pass through and insert some kind of delay or so after this. Is this possible? For the delay I know how to make it, I only need to know if you can build something where only one person can pass through, not multiple at once.