Thermal data is a start but it would be any kind of sensor the school project wanted. It could be dust, gasses, temperature, UV or a range at the same time.
You're probably going to want some sort of external analog I/O board then. Perhaps a gertboard is appropriate, or an Arduino, or something custom. It's something you should think about though as the Pi's I/O capabilities for sensors are pretty limited. This really depends on where you want to take the project - if you're designing the sensors, you can just build an I2C ADC or such inside them if an I2C sensor isn't available directly. If you want to use off-the-shelf or user-designed sensors, you should probably provide a design giving analog inputs. Adding things to the GPIO port may conflict with any forthcoming camera module (?), so perhaps it's not feasible at all?
I'm not sure what use it is to 'use as much of the Pi's processing power as possible'. While the thing is flying you're probably not going to be communicating with it in realtime to do data analysis (though with amateur radio...maybe...but only if it is lower bandwidth than just sending the data). Yeah you could do your analysis on the board, but if you have to wait for recovery to have a look at it anyway, you may as well save on batteries and just do the data analysis later. If the power requirement is low enough that you can do it inexpensively within your weight budget though I don't see why not to try it, it's certainly easy and flexible.