Here's a link to a guide I made. It has the location of every collectible in the game - including Red Bricks, Gold Bricks, Students In Peril, Crest Pieces, and Characters. http://www.xbox360achievements.org/forum/showthread.php?t=24...
A majority of the 100% involves getting Gold Bricks, since there's a few different things you need to get them all. Make sure you actually buy all the characters and Red Brick extras (characters are in the 1st Diagon Alley shop, Red Bricks are upstairs in the 2nd shop). I believe you also have to buy all the extra spells in the 3rd Diagon Alley shop, and make sure you actually beat the 200 Gold Brick bonus level (the one you get Voldemort in).
Lastly, sometimes the achievement takes a while to pop - I ran around a bit before it showed up. Like in other Lego games, you may need to go replay a level and it will unlock after you beat it.
**Please leave a comment/message me if you vote negative or see an error, I'll try to help :)**