In Harry Potter and the Half-Blood Prince, Harry and Ron initially believe they will be unable to advance to N.E.W.T. level in Potions, since Snape requires an "Outstanding" O.W.L. grade and they both receive "Exceeds Expectations." It turns out that Slughorn takes Snape's former position and will accept students with an "E" grade, so they are able to take Potions after all.
However, it seems that out of the 12 students taking Slughorn's class, all of them other than Harry and Ron have the required materials, suggesting that they knew over the summer they would be taking Potions:
"Now then, now then, now then," said Slughorn, whose massive outline was quivering through the many shimmering vapors. "Scales out, everyone, and potion kits, and don't forget your copies of Advanced Potion-Making...."
"Sir?" said Harry, raising his hand.
"I haven't got a book or scales or anything—nor's Ron—we didn't realize we'd be able to do the N.E.W.T., you see—"
"Ah, yes, Professor McGonagall did mention..."
So did all of the other N.E.W.T. students receive "Outstanding" O.W.L.s? Why aren't there any other students in Harry and Ron's position?