I seriously can not see how the school is at fault.
1. The school can not forcefully detain the kid or put their hands on him..
2. Was the kid a Junior or a Senior? When I was in high school, seniors were free to go and leave campus for lunch or when their classes were finished.. In my senior year I left early because I did not have a full day of classes.. (No Need to, I had all my credits ahead of schedule
)
3. Who's fault was it? (the accident, if a 17 year old is not mature enough to stay in school, I doubt his driving was any better)