If I didn't have DD to deal with, an earlier start time would actually suit me better, as it would give me time to drop off DS2 with my friend who will be taking him 3 days a week from January so that I can cross town with DS3 to get him to the SN Nursery on time, an hour's bus ride away in rush hour.
BUT, I DO have DD to worry about, and while in most quarters it is being welcomed at the school, the ones of us that have to rely on the bus service are never going to he able to manage to get there on time.
Afternoon school run times are much quieter by bus - in the mornings, we have traffic destined for the mainline train station to London, plus I live very near a business park, which the morning rush hour buses serve, but the school pick up buses don't - thus making the journey time shorter in the afternoon due to bus routes differences, AND the lack of traffic attempting to get to work.
The school don't liaise with the bus company as the buses are meant to be every 12 minutes. Unfortunately, between 8.00am and 8.35am, there isn't usually a single bus that can get through the station roundabout bottleneck, and 3 turn up at once every day, at some point between 8.35 and 8.45.
(No, I can't walk, despite it bring a relatively 'short' journey of 2 miles, I'm disabled with Arthritis and my 9yo DS2 is also disabled and has limited mobility)
I leave the house between 8.10-8.15am, am lucky to get a bus between 8.35 and 8.45, and either just make it to school on time, or am late, more often than I would like to be, despite leaving the house 40-50 minutes before I have to be there for a 2 mile bloody journey.
Does that tell you how bad the traffic is?!
Leaving earlier won't make a jot of difference, because I can't get on a bus that hasn't turned up because it's still stuck in traffic.
That's why the earlier start times will be impossible.
DS1 has a breakfast club this week, because it's SATS week for him.
We left at 7.45 to be there for 8.30. We got there at 8.45. 
Leaving earlier didn't make any difference, because I can't get on a bus that isn't there!
I can't see why putting the extra 15 minutes on the morning rush rather than the afternoon rush seems more sensible to the school, when they know the traffic issues, and they even have teachers late and caught up in it all quite often.
Seems aimed at making things easier for parents that work or aren't on a very low income, and can afford to run a car, at the expense if those who, for whatever reason (epilepsy and arthritis in my case) can't run a car and are reliant on an abysmal bus service.
Why is an extra 15 minutes, which I have no dispute that they ARE needed, so important to be at the start of the day rather than the end.
I doubt an extra 15 minutes at the end of the day would be terribly detrimental to outside school activities, whereas it will be very detrimental to my DS2's educational record if, due to the morning buses, he is repeatedly late.