Solar Plane, Voltage drop when under a load
Some of the cells have minor damage due to this being a group of high school kids, so that could very well be contributing to the problem.


Very much so! A solar cell is basically a current source shunted by a diode (the latter is what determines Voc). If a damaged (or shaded) cell has a lower maximum current, it limits the current through the entire string, which has a huge effect on Pmax.

Test the cells individually (just connect an ammeter directly across each one), and eliminate any that can't deliver the full 6A.