I don't know whether or not restraining school children in utility closets, storage closets,and cell-like spaces has increased over the years but I think we are paying more attention to the issue. For example, Connecticut has established the Office of the Child Advocate (OCA), which recently issued a report on "Seclusion and Restraint in Connecticut Schools".
The report found that, over the past three years, there were more than 90,000
instances of restraint and seclusion in the state's public schools and more than
1,300 injuries – at least two dozen of them serious.
The report found one child was restrained more than 700 times over the
course of a year. Three-fourths of these children had physical, emotional, or intellectual disabilities.
Furthermore, the report found that many schools are not following the law, and are often
providing insufficient reasons for the intervention and otherwise
failing to adequately document them.