Talk with the person in charge of the school. Is this a public school or a private one? Public schools, by Federal law,.are required to make adjustments to meet the needs of a special needs child. You and the school should not have left him upset for hours. What did you and the school try to help him calm down? My grandchildren, 2 of which are on the autistic spectrum have IEP plans. Individual Education Plans. The school district also provides 504 plans to help ADHD students have success in school. Does your grandson have such a plan? Has he been evaluated to see if he has issues other than just ADHD that cause him to meltdown. Do you have an idea why he's anxious. Has he seen his pediatrician regarding his anxiety.
Does he live with you and if so for how long? Does he feel insecure because he's not with his parents. Has he been abused physically and/or emotionally?
There is a reason for prolonged meltdowns. I suggest you talk with a child therapist to help figure this out.
Because I'm retired, I could go to my grandson's school to both learn about his difficulty and support him. Because he had therapy, I learned how to support him. Do you know someone who could be supportive of you and him?
Perhaps you could tske a couple of hours off work to meet with the school counselor or principal. Would work let you take a couple of hours off.
I do not understanding why the school.had him sit in the office for an hour or more. I don't understand why they haven't helped you to find ways to change this pattern.
There is professional help available if you ask. It will take several phone calls and/or emails to find the right person to help. The school district has an office that is mandated to evaluate and provide treatment for what he needs. There is no charge.