Yes, you can leave school early to attend your music lesson!
The Education Act of the Government of Ontario (1990) provides the opportunity for students in Ontario schools to receive music instruction, often by a private music teacher and to be excused from attendance at school. In Chapter E2, Part 11, Statute 21.2(e) of the Education Act Revised Statutes of Ontario, it states: School attendance may be excused when the child is absent from school for the purpose of receiving instruction in music and the period of absence does not exceed one-half day in any week. (Government of Ontario, 1990, p. 36)