"CONGRATULATIONS! Your exemplary powers of deduction and a formidable knowledge of the inner workings of the magical world reveal you to be a witch or wizard of genuine skill and learning. This first examination has barely tested you."
While most professors at Hogwarts set their N.E.W.T. classes entrance standards at 'Exceeds Expectations', meaning that the students must acquire such a grade in their O.W.L. in order to continue, Professor Severus Snape had set his Potion standards to 'Outstanding', meaning only the very best can continue.