I have an outdoor sand school which I'm very pleased with. I don't jump very big and although it did freeze in the worst of the winter it was fine again once the temperature raised and the snow melted.
I used to have lessons on a fancy sand, rubber and 'fluffy fibre' school and that didn't freeze in low temps but it was also indoors. It was very good at absorbing shock, so you could jump big fences and it stayed flatter than the sand only school.
Our outdoor school is made of 'bog'. It is a lovely sand and a bit of rubber school in summer but in winter it is either frozen solid or deep, boggy and possibly flooded. So to be honest ANY other surface would be a dream come true!