1.) Do you believe them?
I do, if it was done out of religious or racial hatred, they would've added something to indicate that. No action is undertaken of this nature without a very outward sign as to why it was done.
2.) How would you define a joke? Was it a joke?
Where I live, high school kids will fill a paperbag with animal feces and then light it on fire on someone's porch. The idea is that in putting out the fire, the homeowner steps in...well.....yeah.
Something like this may have been what the kids were trying to accomplish, however nonsensical.
3.) What punishment should be given?
Just that which would be the norm for their crime and criminal history.