Neither secondary school-aged DC at SHSG but we are familiar with public transport in the Southend area
Billericay - Southend Victoria 29 minutes then bus no 1 (Arriva) or 26B (First) from the interchange outside the station. The 26B is better because it turns directly into Southchurch Rd, missing out the Travel centre loop. Train arrives 07.54, 26B due at about 08.02 (admittedly not always exactly
but usually fairly close). Lots of SHSG girls use this route.
The 27 is also a possibility but follows a different route from the station and the nearest stop at the school end is about 5 minutes' walk from the school.