You'll need 1/2 heart for each student in your class. You need to play with even numbers, so if you have an odd number of kids, the teacher must play.
Make the heart puzzles ahead of time with the reference on 1/2 of the heart and the key-words on the other side.
Set up enough chairs for all of the kids in your class to sit on.
Now put two of those chairs away.
Place all of the heart halves with references on the floor on one end of the room.
Place all of the heart halves with keywords on the other end of the room.
Gather the kids in the middle of the room. When you say go they run to either end of the room and grab a heart half. Then they have to find their partner and sit down next to them. The two that are last to sit down are out.
For each additional round, take out two more chairs and make sure you have just enough heart halves for each person on that round.
Winners got some Valentine candy. In retrospect, I would not have made the puzzle pieces so obvious. I wanted them to be shouting "Who has 1st Nephi 3:7?" instead of "Who has two rectangles?"