fixes grade-school tests to enforce order

The specification requires that the code return "a sorted list of all
students in all grades". However, the previous test compared the result
against a dictionary, which doesn't enforce that order. This commit
addresses that.

closes #151.

Made possible by great feedback from @Dog and @sjakobi. See this thread:

https://github.com/exercism/xpython/issues/151
Merge pull request #152 from mambocab/grade-school-order

fixes grade-school tests to enforce order