ActiveSync Server Settings
On an Active Sync (Windows Mobile Browser) device, go to Edit Server Settings.Choose Settings.
Try changing that to 1 month (All might be a little tough on the memory in the device. Also look at the Download Size Limit and the Include File Attachments options.
If the device has a storage card there is usually an option to automatically save attachments to the storage card. And most newer devices have the option of encrypting the files on the storage card (this would be important for any attachments with student data on them).