For all those thinking they are missing CGs, you're not. The CG gallery shows 8 CGs per page, so when the last page doesn't have enough to fill up all 8, it looks like you're missing a few. Instead look at the top of the screen where it says 'CG ##/##' this tells you how many you have unlocked.
Example: Haruka has 17 CGs, filling 2 pages and leaving only 1 for the third page. The result is 7 blanks that look like missing CGs, but they aren't! When you have all of them, the top reads 'CG 17/17' and there's only 1 CG on the last page.
So don't worry about missing any. If you finish a route you will have all CGs for that character.
EDIT: Sorry, looks like I messed up a bit. Except for Ran, all of the girls have at least one CG that can be missed if you don't make a certain choice. Also, a few routes have an after-the-credits scene that you may not get if you close the game before the credits end. If you're missing one of these, I can help you out a bit (spoiler marked just to be safe)
Haruka
Choose to share lunch with her.
The last CG appears after the credits.
Mizuki
Look for her at the welcoming party.
Natsume
Don't take either offer, pay for lunch yourself.
When asked if anyone will be nice to us, choose 'Nobody'.
The last CG appears after the credits.
Mei
Borrow money from her for lunch.
The last two CGs appear after the credits.
Rin and Sakura
Follow Ran after everyone goes to the bookstore.