Under the laws of the state of California, there is no time limit on collecting your past due child support.
Also, Department of Child Support Services, provides services to families neding or paying support, with modification and enforcement of orders, including collecting the principal and interest on past due amounts.
I am not aware of any law that prohibits you from OPENING a case with Department of Child Support Services, BUT I have been advised by representatives from child support services that if you do not have case opened before your child emancipates, you will not be permitted to open one then. They have never provided me with the legal authority that allows them to make this distinction, but as I understand it, it is the 'policy' in many counties.
Your situation puzzles me in that it appears your child is under 19 and enrolled in High School. If I were you, I would try again to get the Child Support Office in your county to open a case for you.
If the child support agency will not open a case and enforce your orders, you still have the option of hiring your own lawyer to do the collection for you. You are owed the money and the interest, even if the county support agency will not do it for you. So, don't give up. It is your money and you have a right to get it.