For a long time I have tried to sync my markbook over several devices. Only dropbox has been robust enough to do so – it is well worth my subscription fee.
The issue with syncing across a tablet is that excel often does not work. For me, on Android Kingsoft Office has full functionality of data validation and syncing. On ipad, the only solution I have found (having paid for several apps) is quickedit. It syncs with dropbox easily, and allows me to enter rubric grades. I can do so during lessons or anything similar. It is great, and you should check it out here: