IEPs determine the appropriate setting for a student. Based on the IEP, there may or may not be a choice of setting. I have had children who had to be in a 6:1:1 classroom setting (6 students, etc) but there were not settings like this in the school so the school could modify the IEP with the family. If the question is, what was in this young man's IEP? If he had emotional disturbance but had a Functional Behavior Assessment and Behavior Intervention Plan that was working and the setting was optimal for his academic function, then a mainstream school might have been the option decided upon. However, if he had been having issues with behavior and the lesser restrictive setting was not appropriate (through a revising of his IEP), he could be offered a place in a more restrictive environment--- the family might agree or disagree (then it would go to a hearing) on the setting. Parents/guardians have a lot of power in this.
These procedures are all a part of what the Bush Administration created with No Child Left Behind. Historically, schools had much more flexibility in services offered and placements. But, with NCLB, everything became standardized. IT is difficult to move a child from a less restrictive environment to more restrictive one b/c the idea is that all children should have the greatest opportunity to succeed. The process for moving to more restrictive is one that is done very carefully and deliberately so as to not run afoul of the idea of not having "high enough expectations or providing enough supports to help children succeed." It is not a topic that has come up in our forum but I have lots to say about how NCLB actually creates issues with serving students with ED or CD or ODD.
Again, the IEP is a legal document that must be adhered to. It is not about the child's age but rather what is contained n the IEP, so you can't magically say I want to be mainstreamed when you turn 18 but you can reconvene your IEP and ask for a change, which might require testing, looking at the appropriate setting in the high school, etc. A case in point, I had an 18 year old who was not standardized (didn't take standardized tests or the regents, all work/tests were modified). We had an IEP meeting, he asked to be standardized but because of his IQ the IEP did not change. The student went to the Special Education Unit and requested an administrative hearing. He was granted his request to be standardized. He started to drown under the weight of the work given as now he had to complete things more independently. He asked and was heard.
If the placement is a mainstream school, yes, they can stay until 21 (in my state, I have seen others say 22 in theirs). For many of my students with disabilities (ranging from cog impairment, health related disabilities and Autism), they stay until around 19 or 20. A large chunk of my students are standardized and take the state regents tests to qualify for a Regents diploma. (WE have been proud of our results for our students but our program provides more access to school counselors, social workers, SPED teachers who support students with study skills and their school work.) That said an IEP can be written that says a child will commence job training and get a cert of attendance at age 21. It is all about what is written in the document.
Yes, students can continue to seek to be mainstreamed but their IEP must recommend it. They need ability to complete classes and prove they need a less restrictive environment. This goes back to the protections that are there for the students and ensuring that students are not bouncing back and forth between more and less restrictive environments. Behavioral issues are particularly tough in this vein-- the government requires that behavior assessments, and intervention plans be reviewed every 90 days with data collection and a whole process to ascertain whether the behavior is getting better or worse--not just your feelings as a teacher but actual data for the number of times the behavior focused on occurs within a given setting. ALL required by the government to ensure student protection and effective interventions.
CONJECTURE ---This young man should have had an FBA and BIP with routine data collections to keep the BIP in compliance. My guess (and I have seen with students with ED) is that he held it together for most of the time, especially on the 10 day data collection times. It is often those times that teachers are more consistent with their responses as well. If he had behavioral outbursts outside of the time when the BIP was being reviewed, if the behaviors were not ones that were part of the FBA/BIP, then they would reassess the FBA/BIP and data collect to create a new one. There are formulas that are followed that determine actions taken to ensure that the behaviors that are focused on can be observed, interventions planned, and able to be addressed. IF it sounds complicated, it is even more than you think.