But yeah, this show isn't about answering that question, nor should it be. It's not about these guys surviving and then making a cross country trip to Virginia to meet Rick's group in a few years either.
The nature of a prequel demands more attention to the cause, instead of just turning into another face of TWD. FTWD begins when the media and government will still be active and dedicsting 24/7 to an outbreak of cannibalism.
Since there's no bath salts use/addiction epidemic, that suggestion would be dismissed within a week, but FTWD cannot gloss over cause, otherwise, fans can just stick to the parent series.
Is it bad that I don't like any of them at all and I just want them all to die and come back in the main show so Michonne can chop their zombie heads off? That's the way I was feeling about ten minutes in.

The son's supposed to be a street-living crackhead but they went out of their way to try and make him look like a boy band refugee.
Yeah, it is too typical of modern TV to play the "outcast" who is really the hero angle. His being addicted helps this story how? Merle used drugs, but that did not influence or help his character. Just seems like a preachy, soap opera move.
The girl is just a flat-out walking teen-girl stereotype. ("Like, OMG I hate my mom's boyfriend and my brother and stupid school and stupid life and I'm so outta here!") The boyfriend is the type of "thought-provoking" schoolteacher fantasy that ONLY exists in the fevered imaginations of TV and movie writers, and I just want mom dead on principal for raising the brats and dating the myth.
Yeah--domestic disputes aside, I hope the characters are given some reason why we should care about their ability to avoid zombi-hood.
One thing to keep in mind is that the rumors of walkers or an epidemic clearly was spreading. Many parents were clearly holding their children out of school (and not just on that final half day). That kid brought a knife to school because he knew he'd need protection.
That's where its similar to the 2004 Dawn of the Dead; by the time random characters are introduced, it's clear they are well aware that the dead are eating the living. It should not be long before official acknowledgement is all over the media of FTWD's world.
what are the odds that the listening device/eavesdropping/teacher monitoring thing that the principal was using to listen in on different classrooms will be used sometime in the next few weeks?
From the previews, I believe the big-eared teacher is a walker who attacks the knife kid and the unappealing wife character