What you are missing is the policies of "zero-tolerance". Worried that lenient individuals might fail to enforce rules strictly enough and wanting to make a clear and unambiguous statement about behavior that should not be tolerated, many (especially schools) are setting up "zero-tolerance policies". These say that if you commit a certain offense, then certain consequences will be imposed regardless of mitigating circumstances. In this case, she did (even according to her own admission) create an explosive on school grounds. Now, the "explosive" was extremely mild (hydrogen gas can make a loud "popping" sound), she may not have realized that would be the effect, there was no malicious intent, and no one was hurt. But mitigating circumstances don't count with zero-tolerance policies, so she must be expelled.

The prosecution as an adult for felonies... for that there is ABSOLUTELY no excuse. I only hope that these threatened prosecutions do not materialize.