A lot of girls do use the public transport as most of the school buses cater for Chelt and FOD i think. Most girls walk via London Road, past the petrol station, left into Oxford Street which leads to Oxford Road. That then takes you to the main entrance in Denmark Road.
If you are worried, maybe you could make the trip with your DD a couple of times to get her familiarised with the route.
When she starts in September, I'm sure she won't have trouble finding someone to walk with (judging by the amount of girls we see walking down Oxford Street)