I just played this level, actually. I believe there's a hidden room right underneath Star Coin #3 that contains a door to Star Coin #2 (or maybe it's the other way around?). When I say right underneath, I mean RIGHT UNDERNEATH, it's inside the ceiling that the visible Star Coin is sitting on top of. Just jump at the ceiling there and you'll find it eventually.
It takes a little while to explain, so the best advice I can give is to go to tPeach's castle. You can purchase video tips for a few star coins, which shows how to get a certain star coin, secret exits, etc. The video is listed as World 7 ghost house secret exit. It will show you where the final star coin for the level is. --- Pokemon FC: 3179 7282 0471
You know where the secret exit door is? Get the third boxing ghost in the level to fall down the partial stairs, then lure him to the left. He'll bash through some blocks. Now dispose of him and take that door to go to the secret exit.
On the same screen as the flag, take the door at the bottom, there's your star coin. --- *Casts Doom on everything* You all must now feel the pain that I have felt...DIE!