Let's see, now whats the chance that the boy is lying about the device having been turned off 50/50? My guess is he had the device in his pocket, on and running, listening to music while he should of been listening to his teacher or doing class work.
Lets see, the most likely cause would be that the device was on and being used, which would cause the device to generate heat. If the boy was wearing tight or none loose clothing, that would amplify things. Which would seem to be the case when hearing the nature of said burns. The heat being generated and held into the device very likely either caused some caps or voltage regulation units to blow, which would explain the loud pop.
This applies since while not in use or turned off, the batteries aren't being drained or charged. Batteries only produce heat while being charged or discharged. Either that or the boy would of had to have done something to the device to damage it to the point of making the battery unstable.
Also a note of reference. Second degree burns are not overly severe. They're defined as any burn where the skin is red, irritated and blisters are forming (lay mans terms). So, sort of like if you touch a hot frying pan, and it causes the spot to blister, or say if you've ever had a really severe sunburn (i have, it's not fun when the top 1/3 of your body is sunburned and covered in blisters). Which means there will 95% likely be no permanent scarring.
God, this is like people suing mcdonalds because they spilled coffee on their laps while they were driving, and it was hot. And people say we don't have problems with frivolous lawsuits.