Wow, this is a very intense circumstance! My first thought, due to the severity of the case, is to develop parameters through a meeting with the student, teachers, administrator, parents, and the student's regular therapist and psychiatrist as well as the therapist(s) the student worked with at the mental health facility. With something this significant, it really will require the input and brain storming of a whole team to set up a good transition plan back to school.

These are just some general thoughts and ideas, not advice for how to proceed, as that will solely depend on the healthcare and behavioral professionals and school and school personnel, and the parents/guardians involved to make those decisions for this particular students.  

The transition plan will very much depend on the student's attitude, demeanor, volatility, tolerance, and willingness to participate as well as the resources available in the school. It's hard to give specific and individualized ideas from the outside looking in, but here are a few thoughts:

Teachers could have a universal code word to indicate they need immediate help in the classroom or wherever they are at. This code word could be a quick way for teachers to get help in the classroom by just calling the office and saying the word rather than trying to go into some in-depth explanation while the student may be escalating. This may help minimize the student's reaction if they don't hear a big long conversation on the phone about them.

There should be a specific plan in place for when the student seems to be escalating. A plan for staff and a plan for the student. For example, for staff, when they feel the student is escalating, they call the office, give the code word, and then maybe ask to speak to the student in the hall where the administrator would meet them.

For the student, when either the teacher or the student feels the student is escalating, the teacher calls, gives code word, the student then walks with the administrator or a staff person to another spot where they can deescalate and calm down, like a room in the office or in a counselor's office, etc. The student could just sit there with head down, write, draw, talk, listen to music, etc. This would be a Structured Break and would be the same procedure and process every time.

It will be important that the student is aware of the parameters, processes, and procedures so nothing catches them off guard or agitates them. The more predictable and regular the transition parameters are for them, the better it will likely be for them.

Some student respond well to mentors. This student may benefit from regularly connecting with a staff person they know, like, and trust in the school as a way to express concerns, thoughts, or just build rapport and a positive connection to the school. This doesn't need to be elaborate, just meeting with the staff person once a week for 30 minutes can be a big benefit. And the goal is not for the staff person to lecture the student, rather to be a listener and sounding board for the student with whatever they want to do or talk about.

If this student is agitated by being around other students, than minimizing unstructured time with other students may help to smooth the transition. In this case, you could consider removing the student from passing time. Passing time is very problematic for some students, so just having them pass after or before the bell removes this trigger. And lunch time is another big one. Perhaps the student could have the option to eat in the library if they are feeling agitated or feel as though they are escalating in the lunch room.

Depending on the student's willingness to participate in their plan, you could implement a self monitoring tool whereby the student keeps track on a form when their feelings escalate or turn negative. This may help the student to recognize where and when they need to take a structured break or remove themselves from a place or time. Or it could help them know when they need to prepare to use coping and relaxation skills they've learned or been taught.

It may be helpful to make a list of coping, relaxation, conflict resolution, and any other skills, interventions, and strategies the student has learned or will be taught. This list could be kept in their folder or pocket for reference so they don't have to think about what to do in a given circumstance when their emotions may inhibit rationale or clear thinking. Being able to pull that list out and just pick one could help. And other options could be on the list, like structured time out, go talk to mentor, calm down in office, etc.

If the student doesn't respond well to staff talking to them or giving them directives, it could be useful to sit down with the student and teachers and develop a few non verbal cues or signals. Some students respond better to a cue to take a structured break or to quite down rather than being told through a phrase or verbal directive.

These are just a few thoughts, but a transition plan really needs to be developed with the entire team, both the school team and the specialists from outside of school who will have specific experience and expertise with such a serious condition and issue.

Hopefully someone else will chime in on here who may have had a similar experience or case. Let us know how things go.