Here is my solution that I think it better optimize

School.objects.annotate(
    number_of_class=Subquery(
        Class.objects.filter(
            school_id=OuterRef("pk"),
            is_deleted=False,
            # Add additional filters here
        ).values("school_id").annotate(count=Func(F("id"), function="COUNT")).values("count")
    )
)