Why would a breadboard have a conductive plate underneath it?
Especially when more than one is used, the plastic modules are often mounted on a metal plate for structural integrity.   

Normally the double-sided foam tape which comes on the back of the modules would provide some insulation.  However, it is possible for wire to get pushed out between the plastic of the module and the metal contract strip forming a given row, pierce the foam, and short to the carrier.  

This is particularly true if you have an old or worn module, and smaller 24 gauge wire, for example stripped from old telecom cable.

If you have a single module, you can peel up the foam backing in a corner and see the individual contact strips underneath - hopefully there ins't (yet) a gap between them and the plastic.

With school-issued lab equipment, you are likely to have both well-worn modules, and either the classic multiple modules on a metal plate with binding posts, or an attache case "nerd kit" which functions as a carrier plate - either way, you are more likely to see these accidental shorts than someone using a single new module on its own.