Here's why:
* Collective nouns refer to a group of people or things considered as a single unit.
* "School council" refers to a group of individuals (students, teachers, or parents) who work together as a single entity to represent the school.
Even though it's made up of individuals, we treat "school council" as a singular unit when we use it in a sentence.
For example:
* The school council is meeting today to discuss the new budget.
* The school council has voted to approve the proposal.