Three undead kids and other questions

Three kids are undead in the same class. Which is weird.

Possible explanations?
1) They may not be the same age or from the same class, but all put together so they would have eachother. I don't know about English school system, so I don't know what grade they are, but after 4 years missing I think all undead kids might have been put in the lowest grade of that school level.
Also, everybody is bitchy to them, like if they didn't know them before.
2) School incident, but it's hard to figure out how 3 kids died in an incident without more serious injury.

As for the fact that there are many young undead and a couple of old people:
1) Old people where more easily killable or couldn't eat enough brains to stay alive (if that was even required).
2) Old people couldn't escape their coffin.

Anyway, I really don't understand how so many undead survived in a world with guns. From the start they were greatly outnumbered by the living (only the recently dead rose), it wasn't contageous and it's not like they had a clear mind to make plans on how not to get killed, how to hide, etc.