I suggest looking at Family Fun's website for a craft idea. It's a FANTASTIC website filled with so many great ideas for entertaining kids. If the teacher isn't planning to make Valentine mailboxes to put all the cards in, I think this would be a useful craft for the kids to keep their cards all in one place.
Oh.... I thought I'd come back to your post to add another useful site to go to. They have templates for the projects and many crafts are very easy to do. I also loved using this site when my kids were young to customize behavioral charts and bingo games.