I think more affected are kindergarten because they mostly start school at 9am so school bus run between 8-9am, which is the most congested peak hours. However for primary school, they start school before 8am. As the taxi driver mentioned, it is correct. I'm in wanchai area, the traffic is perfectly fine before 8:15am.
But the worst part would be after school, when most ppl back to school and office. I guess schools may consider to arrange to have short school hours so to avoid late returning home.