I taught several reception classes in deprived areas in the Midlands, where many children lived in houses with outdoor lavatories, and not one child arrived at school unable to use them., even the child terrified of the overhead flush who wanted to use a bucket which was what he used at home. Most of the children arrived in their fourth year because of the general deprivation of the area.
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There were accidents, as the school had only one internal loo for children at that time; the toilet block was in the yard outside, and there were immigrants who did not understand how to use toilet paper, but all the parents had done their best to ensure their children were toilet trained.
Five children not potty trained is a disgrace. The parents in my deprived areas of the past would have been ashamed.
Now I believe it is forbidden to exclude children who are not trained, and too many parents take advantage, citing 'special needs' which is not the case.