The format of grading an essay question is not completely validated.

Create a quiz. Add an essay question to it Attempt the quiz as a student Review the attempt as a teacher Click Make comment or override grade. Try grade the attempt with a range of invalid values (strings, badly formated number like 1..5, numbers that are too big or too small). Grade with a valid value and verify that it is saved. Click Make comment or override grade again. Set the grade back to blank & save. Verify that the questoin goes back into the requires grading state. Now make some more attempts with a few different student accounts. Go to Quiz adminstration -> Results -> Manual grading. Click to grade attempts at the question. Input a mixture of different valid and invalid values into the different grade boxes, and try to submit. Verify that the appropriate validation messages are shown.