If you have a grocery store nearby, go and get yourself a box of the Atkins Endulge Ice cream - they have vanilla and chocolate. It comes in 4 pre-packaged little containers (reminds me of what we used to get in elementary school). It is a full fat, splenda containing ice cream, and I think it has 5 net carbs or lower. It is also very tasty! This way you can go to the party, eat ice cream, and it will shut up anyone pressuring you! Also, it shouldn't mess you up too badly overall and shouldn't knock you out of ketosis.

Good luck!