If you have some basic understanding of how physics works, then the heating of the plate's wrong side should strike you as extremely inplausible - a lot more so than hypotheses about deliberate manipulation of experiment settings. A physical theory should be some kind of quantitative guide about updating probablities, and being such it should be able to give you very low probabilites for a class of hypotheses. The teacher's trick drew attention to the fact that the students markedly did not use physics to generate quantitative answers. So, per my reading, the focal point of the experiment wasn't suspending trust in people.