ETA - just noticed you mention Westcliff
station - nearest station for WHSG is Chalkwell!
I'm going to assume that you are enquiring in advance of making possible CAF decisions, rather than from a perpective of having chosen WHSG for your DD who now finds that that she doesn't like the available travel options?
One way of doing the journey would be to get off the train at Southend Central station, then get the bus back towards the school from the travel centre, which is a few minutes' walk from the station. You then have the option of using the 21 or the number 29 Arriva service, which stops on Kenilworth Gardens outside the Westcliff schools.
What have you got against the 21 - other than it gets rather full? All the buses do at 'school' times, I'm afraid.
A huge crowd of WHSG / WHSB / Tommie More kids seen to have no issue with walking over from Chalkwell station, though