My friend is a teacher and the kids she teaches definitely deserve Valentines - and so does she! Bookmarks are admittedly very unexciting, but the kids in her class aren't allowed to have candy (the horror!), and who knows what else they're not allowed (I think she told me once she's not allowed to use Febreze), so bookmarks seemed a safe bet, and as Ms. Leone says, "if they think you made something especially for them, they'll love it no matter what". They're also good last minute, and simple to do if you have a color printer and pretty ribbon, perhaps glitter for the more adventurous. These are the ones I made for her class, followed by some printable, customizable ones. There are probably tons of examples of punny Valentines on Pinterest and the rest of the internet (a snail saying "I slow love you" - that only barely makes sense. . . ), but some of those might be a little creepy coming from a teacher, and I didn't want to reveal my uncoolness by having no idea what kids think is cool, so I kept mine pretty simple.
To make your own bookmarks for kiddos, here are some fun, free fonts to start with: Always In My Heart, Gold Panda, KG LET HER GO, Penelope Anne, Simon Script, Words of Love. Below are some fairly plain ones I did that would work for a variety of ages. Just print on card stock, punch a hole in the top, and tie with ribbon.