You bring aa blank pistol into a bank and attempt to rob it, you will be charged with armed robbery! It’s not the fact it wasn’t a real firearm, it is the fact he wanted everyone there to believe it was.
You are engaging in the logical fallacy of denying the correlative. Robbing a Bank with a gun or a banana is irrelevant to the actual crime.
The case in question is not related to your statement. The teacher used questionable methods to scare his students for whatever reason (most likely a valid one). The kids were never in any physical danger. Federal law prohibits him with being charged for firearms since federal law says that what he used cannot be described as one.
While I don’t agree with his methods, how the teacher is reprimanded is the choice of the school district...not by public opinion. The police have no authority here since no laws were broken. Nor is jail time fitting of his actions since no crime was committed.
Next I suppose you will be calling the police on your neighborhood kids for playing cops and robbers while firing cap guns or airsoft weapons.