Grade School

Given students' names along with the grade that they are in, create a roster for the school.

In the end, you should be able to:

• Add a student's name to the roster for a grade
  • "Add Jim to grade 2."
  • "OK."
• Get a list of all students enrolled in a grade
  • "Which students are in grade 2?"
  • "We've only got Jim just now."
• Get a sorted list of all students in all grades.
  Grades should sort as 1, 2, 3, etc., and students within a grade should be sorted alphabetically by name.
  • "Who all is enrolled in school right now?"
  • "Let me think.
    We have Anna, Barb, and Charlie in grade 1, Alex, Peter, and Zoe in grade 2 and Jim in grade 5.
    So the answer is: Anna, Barb, Charlie, Alex, Peter, Zoe and Jim"

Note that all our students only have one name (It's a small town, what do you want?) and each student cannot be added more than once to a grade or the roster.
In fact, when a test attempts to add the same student more than once, your implementation should indicate that this is incorrect.