The kid is responsible. If your child cannot be contained, either by his parents or the school, he brought on his own fate.
I've seen kids skip class before and when teachers try to stop them, I've seen them just walk off and ignore them. It is not a teachers responsibility to physically restrain a teenager and make them stay in class, if it was, I'm sure they would lock the doors and not let them out until 3pm.
Those parents are either too greif stricken that they are just looking for someone to blame, or it's a cash grab which would be disgusting.
If anyone votes that it's the schools responsiblity, I'd like to hear their justifications as to why.