I dont know if this helps but here goes. We moved to Steeple View to be in the catchment for The Billericay School (which, apart from the Grammars which we hope our children will be bright enough for, is our chosen comprehensive). The people who we bought our house from sent their children there and were very happy with the school and pointed out the school bus that picks up nearby.
I see this bus every morning as I take my kids to Primary and Ive been horrifed with what Ive seen - kids "bundling" younger ones, "gestures" out of the window, books thown the length of the bus - just general mayhem and its totally put me off! Although its 2 years until my eldest is in even in Yr6 I WILL be tackling the school about this nearer the time.
Since noting this behaviour, Ive actually made an effort to see what happens on other school buses that I see everyday (including the ones to the Westcliff & Southend Grammars) which is waiting at Laindon Station when I drop my husband off in the mornings. Usually the "mayhem" is upstairs and Ive seen the Grammar buses pretty lively, although probably not as bad as the goings on Ive witnessed on the Billercay School bus!
Having 50 or so unsupervised (for the most part) kids in a confined area is always going to be a challenging environment, however if bullying on these buses is a real issue, its something that the school needs to tackle.
Its my main concern about choosing a senior school for my kids (the transport) as we dont have one that is within walking distance and going to be a hot topic when I do the rounds of open evenings next year!
All I can suggest is contacting the school to find out their take on it, and possibly following the bus yourself a couple of times to see if you can witness some of what goes on (maybe a bit extreme, but I know Id do it LOL!).
My friends son goes to WHSB and gets the bus from Benfleet Station (just a normal bus not a school one) and she says he loves it, and there is never any trouble.