In a typical scenario, the parents’ responsibility to support a child ends at the age of 18, when the child has reached the age of majority. However under Ontario law there are some exceptions, such as where the child has special needs.
Another exception can arise if the now-adult child continues to pursue higher education past the age of 18 — provided the child is still enrolled full-time in a post-secondary education program and is considered “dependent”.
In the described question, the child would not meet that threshold test, since she is taking only one course in high school, and not the required full course load relating to the pursuit of her higher education.
However, even if she were enrolled in a higher education course and attending full-time, her entitlement to support would not be automatic; it would depend on numerous factors including the child’s age, academic aptitude, goals and overall best interests. It would also depend on your situation as her parents, including your respective financial circumstances.