Out of curiosity, how would the students actually go about making their grades private. Free github accounts don't actually get any private repo's, so aren't you effectively requiring a student who wants to maintain some secrecy to pay extra?
2. The repos could be maintained on any git server, so bit bucket would work just fine.
3. Even if the students had to pay for github private repos, the price would be far less than even the cheapest textbooks.