There's really two issues. The first is the hard core fascist way of thinking where anything not compulsory is forbidden and anything not forbidden is compulsory, so her mistake was doing anything on school grounds with even a minute quantity of creativity. School is the place where human spirit is intentionally by design ground down, turn them into uncreative uneducated automatons so they can appreciate tightening bolts on the factory assembly line for 12 hours a day. Oh wait that's been obsolete for decades. Oh well who cares there's money to be made doing it and metric goals to achieve and tests to teach to.
The second failure mode is its really pretty stupid, just generally speaking, to randomly mix stuff together, and randomly screwing around mixing stuff isn't chemistry, so lets be honest about what really happened, she read about how to blow stuff up in a fun mostly harmless manner and thought it would be fun to try at school, and it worked and was safe, but she got caught. Its mostly harmless so rather than convicting her of some kind of terrorism felony they showed some mercy (or the police properly and rightly told the school to F off) so her story is being rewritten with lots of randomness added. There may be some interaction with profound ignorance of chemistry on all sides of the story from the kid on up.
Link to police report: http://www.scribd.com/doc/138927259/Wilmot-Arrest