Can I sue a school board if my child was injured due to a lack of supervision in Ontario?

Yes, in appropriate circumstances. School boards and their staff owe students a duty to provide reasonable supervision, suited to the children's age, the specific activity involved, and any known risks, effectively standing in a position similar to a careful and reasonably attentive parent while students are in their care. A claim can succeed where an injury genuinely resulted from inadequate supervision - staff absent when they were needed, a known hazard that was never addressed, or a student with known behavioural risks who was not reasonably managed.

Schools are not, however, held to a standard of preventing every possible accident; ordinary childhood mishaps that happen despite genuinely reasonable supervision generally do not establish a breach of this duty, since courts recognize that active children will sometimes get hurt even when supervision is entirely appropriate. What counts as reasonable supervision in the circumstances - given the child's age, the nature of the activity, and any known individual risk factors for that particular child or situation - is a fact-specific question, which is why records of supervision plans, staffing levels, and any prior warnings or incidents involving the same hazard tend to be central evidence in these claims.