Dummy key... Well, I guess that works to troll a thief. Otherwise it would be all the same to just keep the only key in your pocket and leave nothing behind in the teachers' office.
Also, it was a hilarious scene to see someone plead in defence of a corrupt politician. In the majority of countries corrupt politicians are loathed by everyone, with the exception of the criminals benefitting from the actions of dirty politicians. Japan is the country where the prime minister was publicly assassinated not that long ago, so seriously Tsubaki should sooner expect pigs to start flying than anyone wanting to help her rotten father.