Do you mean Westcliff High School for Girls?
Being nearer a station on the c2c line - eg Upminster - would help; loads of WHSG / WHSB and St Thomas More pupils come in on the train to Chalkwell (15 - 20 minutes walk to the schools depending on how fast your DD walks
. Bit of a hill up from the station to Leigh Road, though). A lot seem to arrive on the train which gets in at 08.02, so the one which would be the 07.34 from Upminster. An advantage of the train over road travel is that the trains usually keep running fairly normally when it snows.
I hesitate to ask, but did you (or more importantly your DD) try out the journey before applying for a place?