ok being a ex-teacher here are my suggestions:
make sure you have an exact schedule of events (down to the minute), kids need this structure! Believe me you will be thankful for it later as well!
Start with the most structured activities first, then move on to the crazier ones towards the end when they get antsy.
(wait to do exchange of valentines till LAST, it will eliminate a lot of chaos)
Timed stations (10-15 min) at each, of smaller groups of kids rotating doing various activies is also very effective.
-Math Fun Station - kids do multiply/divide facts using something with redhots/converstaion hearts/ or valentine's math worksheet
-Valentine's Mailbox creation station
-teamwork unscrable words: http://www.abcteach.com/free/u/unscramble_valentines.pdf
Here's a few games:
Valentine's BINGO a winner for every party and can be done in a large group
Small cooperative groups, time race to put a "valentines theme" puzzle together
Decorating treats - have precut sugar cookies ready - kids ice and decorate thier own valentine cookie!
If you are going to do something where kids are going to be "out" have a quiet activity they can do such as a wordsearch, crossword, or coloring activity
Just Google! its awesome what is already out there! Good Luck to you!