Ada's friend Rita - surname unknown

Does anyone recognise her? Postcards sent to Ada by Rita, dated 1916 and 1917, show that they were friends, and it appears that they both attended St. Sampsons Salvation Army Corp at that time.

The photo was taken by Tupper of Victoria Rd, St Peter Port, Guernsey.

It is signed on the back signed on the back "to dear Ada from Rita, July 1917, age 21".