It appears as though your students are attempting to gear up their base to drive too fast. The ratio they are currently using is too fast to use with only 2 motors, and I wouldn’t recommend it with 4 motors either. Also, as the robot is beginning to become finished, those 2 motors would be carrying too much weight for the 2 motors to handle without stalling.
I would recommend that your students use 4 motors for their drive and for them to gear the base to a slower speed. There’s no point in having a fast robot if it’s going to stall half way into a match
First of all, the wheels you are using create a LOT of turning scrub (http://goo.gl/bpCXjB). Because of this, your robot will struggle to turn in an efficient manner, and a LOT of energy is wasted from the friction between the wheels and the field. I strongly recommend purchasing Omni wheels from the VEX website if you want to solve this problem.
Second, you are using a gear ratio that is WAY too high for the number of motors you are using. I can’t tell exactly what the gear/ sprocket ratio is, but I can tell it is too high. Because of this, your motors are tripping the PTCs.
Third, you need to use more motors on your chassis. Two is simply not enough if you want to win. I would say that a team should use at least four to be competitive, otherwise your robot will be too slow.
Fourth, you have to support the sprockets on both sides. I can see from the photos you are not doing that, and it is creating a LOT of friction.
TL;DR–> You need to rebuild your chassis to prevent if from burning out. What you have right now is simply not good enough.
