It is silly that they actually arrested him after they found out what it is all about.
Although enough has happened in the past to make it a fuzzy case IMO.
He brought something to school that should not be on school grounds, period.
He at least could have asked his teacher first.
It was not 100% clear what it was, so yes you can imagine someone uneducated like an english teacher to get nervous and take it serious.
There were enough incidents of kids bringing a fake sometimes selfmade gun to school, it looked like it was real but it was not, you think that would be serious enough to call the police? No? Well it also has happened that kids brought real guns and killed people in school, that happened in the US and also Germany and probably other countries, a lot of innocent people died.
Only thing I find ludricous is that they really arrested him after they found out it was nothing like what they thought it was. The first reaction of the teacher is ok, better safe then ever being sorry and have to apologize to parents when their kids died.