Unfortunately there have been children in school, in Early Years in particular, but sometimes in KS1, who are not toilet trained for a number of years. It is certainly a more recent phenomenon, because when I started teaching in the seventies there were no such children, and almost no accidents, even with the youngest children. By the time I left some nine years ago, it was no longer uncommon for children to start school untrained. I think disposable nappies has caused the change, with parents no longer needing to wash and dry nappies daily, the pressure to toilet train was diminished.