Record ex-pupils as they walk around the grounds and talk about their time at the school, what sort of games they used to play, stories about the way that their school day was structured etc.
You could start with parents and grandparents who are local, walking with them and recording outdoors. For less mobile people, you could run 'story circles', getting together in a small group and recording them with a table microphone.
Stories could be categorised into different time periods, such as pre/post-war, different decades etc.
Ask them questions that could lead to more mediascape content- what their favourite music was, their favourite place in the school or its surroundings etc.