A study permit is usually required if your program is longer than 6 months. Most applicants must include a provincial or territorial attestation letter (PAL/TAL) with the application, unless they fall under an exception.
Typically, your school (or province/territory process) helps facilitate the PAL/TAL after your admission steps are complete. Timing can vary.
Minor children who want to study in Canada for 6 months or more generally apply for a study permit before entering Canada. If a minor child is accompanying a parent who is applying for a work or study permit, the child may not need a separate LOA for the child’s study permit in some scenarios (case-specific).
Minors already in Canada may need a study permit depending on province/territory rules and access to services. Some minors can study without a study permit if they meet specific conditions (for example, a parent is a Canadian citizen/PR, or the family has protected person/refugee claimant context).