Thank you so much for this, Chimera-ma,
Really useful. I'll check out Trinity School routes. I do think WoK could be a little more helpful about this, if I'm honest. Otherwise everyone will chose Tonbridge, regardless of whether they live near Sevenoaks or not.
I live very close to the annexe site and my nearest bus stop is one serving the site.
There are many school bus services available to the annexe already. They started up last year when Trinity School, which shares the annexe site, moved from a different temporary location to portable cabins adjacent to the construction site. That school is now in its newly-completed permanent building and the number of buses serving the school has increased and must surely increase further with each year's new intake, as the school first opened (elsewhere) with only Year 7 in 2013.
So my recommendation is to check travel routes for Trinity School and the same information will apply to the grammar annexe.
I don't know the specifics of the school services (my DS goes to TWGSB) but I have, for example, seen a 431a, operated by Go Coaches, go direct to the school. You might like to check Go Coaches and Arriva local bus services from Sevenoaks station and the town centre, such as the 308 and 452, but from memory they don't tend to operate until about 9:00 AM - I suppose they have a shortage of vehicles until then.
The nearest train station to the site is Bat & Ball, one stop from Sevenoaks station on a Thameslink line which runs to St Pancras via Bromley South. Bat & Ball is about a 12 minute walk from the school. It is currently unstaffed and services are unreliable, but there are plans to upgrade the station as part of a Thameslink refurbishment programme and Transport for London is looking to take over the line when the current franchise ends in (I think) 2018.
Sevenoaks Town Council are working on plans to build a new council chamber/community centre next to Bat & Ball Station, having apppointed architects after a competition last year. They propose having a cafe which will also serve commuters and make the station more user-friendly in bad weather.
Hope that helps a bit.