Liabilty for non majority aged child

What is the name of your state? Missouri

My stepson is 16 yrs old, and will be 17 in July. In Missouri, it's perfectly legal for a child to leave home at the age of 17. I have searched endlessly for information about the subject of legal liability for parents of children who choose to do this, but have been unable to find information.
My stepson is troubled. I have zero doubt he will quit school, as he makes no effort to pass. His 16 yr old girlfriend is pregnant, and is also troubled. Stepson is on probation initiated by his father and I to control his bad behavior, which will expire when he turns 17.
His father and I lose sleep over this predicament. We just need to know that if stepson gets into trouble, can we, as parents, still be held liable for his actions until he turns 18?
Thanks in advance for any advice.

Please let me clarify a couple of points.
By perfectly legal to leave home, I mean that local law enforcement will not force your child to return home, nor will school officials. As stepson doesn't plan to attend school anyway, school won't be an issue. I realize that in all 50 states the age of majority is at least 18. But if the State of Missouri will not enforce returning the runaway child to the home of the parents, how can the parents be responsible for any bad acts of the child until the age of majority?