In the Guildford section of Kelly's Directory of Surrey, 1930, the only girls' school mentioned is the County Secondary School for Girls, Portsmouth Road and Farnham Road (page 202). Archbishop Abbot's School, North Street, seems to have had strong traditions in music, but I think it was for boys only.
The Farnham section (page 175) notes the Girls' Grammar School, West Street, established 1903.
Guildford Cathedral would, potentially, have been a source of support for budding vocalists - at least, for boys; I don't know about girls.