Sync grade-school docs. (exercism#2681)

This removes the bonus section and last two paragraphs about protecting
internal data from external changes and sharing thoughts/comments.

exercises/practice/grade-school/.docs/instructions.md

@@ -1,7 +1,6 @@
 # Instructions
 
-Given students' names along with the grade that they are in, create a roster
-for the 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:
 
@@ -11,28 +10,12 @@ In the end, you should be able to:
 - 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.
+- 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"
+  - "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?)
-
-## For bonus points
-
-Did you get the tests passing and the code clean? If you want to, these
-are some additional things you could try:
-
-- If you're working in a language with mutable data structures and your
-  implementation allows outside code to mutate the school's internal DB
-  directly, see if you can prevent this. Feel free to introduce additional
-  tests.
-
-Then please share your thoughts in a comment on the submission. Did this
-experiment make the code better? Worse? Did you learn anything from it?
+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.