The kid probably has mental health issues. It's pretty clear the Mom has more issues than Newsweek.
He probably lived one life at home and a different one at school. It happens ALL THE TIME. How many people even know their daughters are knocked up until they look like they're about to pop?
I suspect you will find out he was either a bully or bullied (more likely) at school. At home he was probably as normal a kid can be in the eyes of Ma and Pa, because I suspect their idea of normal is a bit... skewed.
Nobody is going to let a person handle a tool with such potential finality as a firearm if the think they make get shot. Most people don't want to die. I don't believe the parents thought the kid was a risk. It'd be like handing your alcoholic belligerent, angry, and mentally limited uncle a loaded 1911. Might not be a big deal or the the last thing to go through your mind might be a 230gr freedom pill. You're simply not going to take that chance.
I don't really believe in the focus of locking up firearms for a teenager. At that age you either grasp the power a firearm has when you wield it, or you don't. If a kid can figure out why it's bad to play with matches over a trash can next to the curtains, they can figure out when an expedient projectile leaves a weapon it can be just as catastrophic. I managed to learn the finality of a firearm at a young age simply by my Dad shooting a junk car with a 44 Magnum. I knew my entire life where all of his firearms were, I handled them loaded without him around, etc but I never really shot them until I was 10. The most he ever had, and still has, was a briefcase he kept locked with said Dan Wesson 44 magnum in it. I simply didn't care that much. Big yip, I can poke holes in things is what I thought. It was way more fun to blow up rocks with an acetylene torch and all sorts of other equally dangerous tools.
The school system probably knew if they turned the kid in for crazy, it was going to haunt that kid for the rest of his life. That shit you never escape. It might not have seemed that bad to them. Especially if they saw the reaction of Ma-n-Pa (pure speculation on my part) being batshit crazy that their little Ethan was nutjob... the school may have figured if he went home he was going to end up with a black eye if he went home. However, if it's bad enough to call Ma-n-Pa in, it's worth keeping him in in-school-suspension until he can get evaluated.
And yes, the kid on Dexter's name was Ethan. And yes, it was on the air before Ethan went on his shooting spree.