And what percentage of them would actually be able to do anything with it? Given that the Columbine kids were smarter than all the rest of them over the last 15 years and they couldn't get it to work, and given that there have been several other attempts to detonate IEDs here since the Oklahoma City bombing that have mostly failed, with the biggest success being a 3-person kill.
If there were no guns (not what I'm arguing for) there wouldn't be 0 of these attacks, but there would be a LOT less of them. Timothy McVeigh had extensive military training, a high IQ score, and plenty of disposable income and still needed a lot of things to go his way for everything to work.. How many of the recent gun attackers meet that criteria?