We don't know much yet about the story of each "level" (nor about the detailed gameplay) but I think the game shouldn't only be restricted to a kid (the player) versus many zombies.
I mean, it would be cool, if the player randomly met other kids in the school during his adventure. These kids would try or to escape zombies or try to kill them too. Maybe these kids could be NPC, maybe other characters from the Character Selection screen.
Randomly meeting them in the game would allow the player to have various gameplay types :
- in a level you could have to rescure a group of frightened kids and lead them to a safe place
- you could be helped by another kid (a NPC) to fight big enemies (like in the game Another World : there was a big monster who helped you from time to time)
- this would add "life" to the game (after all the story is in a school... If the game is only about killing zombies from the start to the end, again and again, without alternative missions, it could be borring)
Of course, most of the time, the player would be alone against zombies.
Maybe if there are single player/multiplayer modes that are different from each other, in single player you can have a "BFF" that assists. Then the question is - should the player chose their BFF or should it be hard-coded in as a complimentary character? Could you apply upgrades/equipment to your BFF?
I guess for the lower tiers, to keep it simple, it'd be the complimentary character that gets the same bonuses you do or an equivalent "upgrade score" if there are different base stats for each class.
I do like the option of having temporary allies or quests to save or assist other kids, too.
Sounds like a plausible idea. Wouldn't mind playing the role of escort and all that jazz. Maybe a few little side events of holding off small waves while someone repairs one of the barricades or something along those lines.