Middle and High School students want to party. In fact, many times when we ask them why they drink they report that there is nothing else to do, or they want to relax. Teen parties can be fun for teens, and still meet your considerations about being safe and sober.
Resist your inclination to control the process. Let your kids learn how to entertain by guiding them with curious questions about how they might want to handle things like food, beverages, party crashers, and dangerous choices. Share with them what the limits might be in terms of budget or numbers of guests, but let them get the ball rolling. Be judicious about reining them in. This is all a part of them becoming successful independent adults.
There are some useful websites to consider when planning teen parties. Check these out, and share them with your teens if you think that the kids are open to some suggestions.
How to Do Things.com — Another resource with some useful ideas
Remember none of this guarantees that nothing illegal will be happening during the party. But, it goes a long way toward acting responsibly and encouraging your children to do the same.