Well the likeliness of every character getting a chance to shine and show that despite being a high schooler, they can survive isn't likely to happen when you have too many people who are just great alround. Like for example, a character who's great at everything enters the school, leads everyone and stays alive no matter what happens. If everyone else just follows this person and believes they can do anything so they don't have to worry, all of them become just basic sheep while the leader is the Shepard. That doesn't seem interesting storywise since individuals wouldn't be able to show off their talents or add to helping everyone escape since they wouldn't need to. Students are less likely to be amazing at everything because if that's so then what's the point in them being a student in the first place? At least this way where everyone has at least ONE bad area (which should be highlighted in the Skills section of the CS) then it shows that there is no one perfect leader, everyone has to pitch in if they want to survive o3o 

If you've seen any Zombie show/game/movie, the characters have areas they excel in and others that they don't, but that all a part of the story, it adds to the horror atmosphere. Why would people be afraid of zombies if the characters they've seen all easily overcome them? xD Like in the Walking Dead game, that right there is such a good game with great characters. None of them are perfect, nobodys perfect in general. This adds sweetness to every success, finding food and stuff, things that we take advantage of, become such a big achievement. However no one would feel that way if they were just following a perfect leader who is able to do everything o3o