First off, this is unpossible. Maryland has relatively strict gun control, and in any event, you can't legally bring guns into schools.
The kid was reading Inspire, which is AQ's english language online magazine. Based on the wiki for it, Inspire focuses on recruiting so called "lone wolf/homegrown" terrorists. It has lauded the attack on Ft Hood by Nidal Hassan and pointed out numerous times that there are many soft targets which are undefended. Luckily we've been very successful in stopping these attacks due to law enforcement and good intelligence.
Given that these plots often pick target areas where the attackers know that nobody else will be armed, and the Al Qaeda propoganda points out the advantages of such target sets, it seems fair to say that the enemy has cracked the code on victim disarmament zones. Just something fun to think about, especially as DW and I both often have to work/travel in such areas, given that even Pres Obama is worried about "lone wolves."