Choose some landmarks in the schools grounds and use them as sites for students to attach opinions and comments.
To do this will mean that you need to provide recording equipment for the children to take home. Ideally this would be an easy-to-use, good quality sound recorder. (Most people prefer not to see themselves on video but you could use a video camera with the lens cap on.) PDAs can be used for recording very short sound-bites.
Think about the output: if you make a web version of the mediascape (such as a map which includes hotspots for the sound files) it could be accessed by the family from home. (This could also be useful in other 'remote' ways: for example children who are in hospital could have remote input to a shared class project.)