English YA book girl gets first horse, struggles in school

Looking for a paperback I bought in England circa early 1980s, a girl gets her first horse and then struggles with her grades in school and may have to give the horse up. I recall a scene where she picks cress for tea, and another where she is so short of time that she has to do her algebra on the bus to school.

Possibly Fly-by-night by K. M. Peyton? It's been a long time since I read it but I clearly remember that it showed how Ruth's dream of owning a horse leads to a very difficult reality as she struggles to manage all her responsibilities.

>2 Sakerfalcon: Thank you, I will take a look!