Change grade school tests and example to encourage idiomatic Elixir list building

@pminten reminded me that lists in Elixir are implemented as singly
linked lists and as a result it is O(1) to add an item to the beginning
of a list, while it is O(n) to add an item to the end of a list.

I updated the tests so that they do not care in what order the
students are added to the list, and updated the example to use the
[ head | tail ] form of adding an item to the head of a list.

I'm an Elixir newbie, so if my change to the example is not ideal,
feel free to correct me!