Well, first, the SP for secondary school specifically prohibits the international student to work. Whereas, SP for post-secondary allows the student to work part-time (there is a temporary policy right now that allows them to work full time while studying). Also, the SP is tied to specific DLI (at least for post-secondary).
According to IRCC:
https://www.canada.ca/en/immigration-refugees-citizenship/services/study-canada/change-schools.html
Moving to the next education level
If your study permit is still valid, you don’t need to change or apply for a new permit if you’re moving between school levels. This could be:
- primary to high school
- high school to post-secondary
- post-secondary to high school or
- any other move between school levels
If your study permit will expire, you need to
apply to extend it.
If your permit has already expired, you must
restore your status as a student at the same time as you apply to extend your study permit. You must apply to restore your status
within 90 days of losing it. You can’t continue or restart your studies until you have status as a student again and we’ve extended your study permit.
However, I just decided to not complicate my son's SP application and just submitted a new one.