This is a really interesting question. Mechanistically, it might all come down to flavor science/biology instead of simple chemistry.
However, I did find a really cool old paper that described how adding salts such as NaCl to acidic HCl solutions can increase the acidity (further lower the pH) of the solution.
The mechanism they suggest is that because the Na+ and Cl- ions are hydrated when dissolved (i.e., they are surrounded by a "shell" of neutral water molecules), those ions effectively end up competing with H+ (or H3O+ or however you want to depict solvated protons, which is a complicated question in itself) and therefore the acidic protons are less "protected" by the surrounding "shell" of water molecules, which increases the acidity.
I would be surprised if this is the answer that the teacher was expecting the 7th graders to come up with, though.