My child was put into the German/French stream when she entered her secondary school. She had been learning Spanish since Y4 (relatives & friends live in Spanish speaking countries) and we have kept that up privately. She is very lucky in that she has extremely good linguistic ability and she has decided she would like to do Spanish at GCSE.
She cannot do it through her school because she's not in the right strand. Also, she wants to do German and Latin so that's her languages choices taken.
Her external Spanish teacher believes that she would be on target to achieve an excellent mark in GCSE in Y10, so a year early, and we are exploring our options to make this happen but I believe she would need to be entered as a private candidate.
I have not yet spoken to the school regarding this possibility but have other parents been able to put their child forward for an extra subject through their own schools or would we need to find another venue. I am finding it difficult to know what to search for when I google.
For context, she's on an 8/9 flightpath for all academic subjects, I think if she were struggling I would not even consider this as the extra workload would not be fair.