This is not an official perspective, and I could be wrong about this, but my experience with IEPs is that they govern a school's provision of educational services while a child is in school. My assumption would be that a school would not be obligated to re-enroll a student into school after the child has graduated simply because the child had an IEP at one point. My assumption would be that the IEP most likely expired at the time of graduation.

An additional consideration would be the obligation of the treatment facility to follow an IEP, even if valid. For example, if the school is private and receives no government support, I'm not sure a school be obligated to honor an IEP created by a public school. Furthermore, even if they were obligated to follow an IEP, I'm not sure they'd be required to re-enroll you in school because you once had an IEP.

In any case, it's unfortunate that you want to pursue further education and aren't being allowed to by the facility, assuming they have the space, resources, and capacity to provide you with a valuable experience. I'd wait for additional responses on this forum by someone who may know the exact answers to your question, and possibly meet with a social worker/counselor you may see. Good luck, and it's great to hear you are still wanting to continue your education!