It's only an apparent lack of consistency if you use the two items to create the context in which to judge the responses. Here's the more likely scenario:
* There was no punishment forthcoming in the accidental shooting case because, after determining that there was no intent involved, and that it was, in fact, accidental, no punishment was deserved. Since there was no possibility of the parents suing themselves for damages, or that affecting the greater population, it got left at that.
* The incident on school property was punished because A) There was clearly an intent to make the explosion and B) it was on school property. That means lots of children who could potentially have been harmed, and that means lots of parents who could potentially sue the school system. Even if no one got injured, the potential for injury might be enough to have a jury in a civil suit feel that the plaintiff is "entitled" to "damages."
In that second case, everyone suffers. If the school has to pay out money to one set of irate parents, other suits will likely follow as everyone thinks they need to "get theirs," too. But guess who is on the hook? The school district, funded by property taxes or whatever they use in Florida. Thus, the community is the ones putting up the money to pay out to some jack-ass members of the community who want to take advantage of the situation. Being able to say "Look, no one got hurt but we have dealt with the perpetrator to the fullest extent of the law" goes a long way in staving off civil complaints, or having them be validated by a jury if someone thinks of doing it anyway.
It's all a CYA move. Does it suck? Yes. is it fair? No. Is life fair? Hell no. But unfortunately, we live in a chicken-shit, overly litigious society where these things happen. My mother is a public school teacher and the district where she teaches has had to deal with things like this in the past. "Science" wasn't involved, but the schools have been sued in the past, and in one incident $5,000,000 was awarded for "negligence" by the administration because two guys were fighting over a girl and one went through a plate glass window. Public schools are strapped enough for cash as it is, and losing $5,000,000 when you're already in budget shortfalls due to declining real estate values (and thus property tax revenue) is tough.
I would wager anything that was what they were concerned with above anything else.