this hinges on good faith and ignorance of the law. you can legitimately arrest suspects under the auspices of 'suspiscion' of a crime (eg, murder). Whereas false arrest is an arrest contary to known fact.
In cases of bomb threats and security breaches, qualified immunity is going to provide significant leeway when dealing with IEDs. Since most high-schools have strict anti-weapons rules, you can get arrested for a swiss army knife. Again, this is not the same thing as an apologia for a perp walk of a 14 year old. But if you want to see a "false arrest" look at what happened to the Blake last week in NYC. That was an unreasonable takedown of a private citizen with no effort made to establish identity; and a violent arrest for a non-fleeing suspect in a non-violent crime, etc.
In this case you are dealing with an "IED" vs an "Improvised Timekeeping Device", and since the former often includes the latter as a component part, its not completely baseless. This is why you cannot fly with liquid water on an airplane--the security forces cannot distinguish (on-sight) "legitimate water" in a clear bottle from several chemicals (aslo clear) than can be combined into an explosive IED that will puncture a pressure hull of an airliner. If you insist on taking your water through security you will get it removed or be arrested.