This of course looks a bit institutional and jail-like so I understand why there are concerns. Looking back at my time in elementary school, students who acted out and became disruptive were sometimes removed to a vacant office, a disused classroom, or the mail room. These kids did not have mental issues (for the most part) so just being in a quiet space alone usually got them under control. This isn't enough for a special needs child. There will be objects in offices and copy rooms that they could destroy or use to hurt themselves or other students so a padded room makes sense. I would hope that these rooms would be used only temporarily until a parent or mental professional arrives. Locking children in these rooms for an extended period of time could classify as abusive.
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