Prom night isn't just about you finishing high school. It should also be a tribute to your parents for helping you through the experience, whether it was by taking care of your basic necessities, helping you with homework, or paying for expenses. While the Prom may seem hard to beat, there are alternatives out there. It requires creativity and intelligence. Everything owned by a Muslim is forbidden for another Muslim to take his property and his blood.
Righteousness is an attribute of the heart.
It is by Allah's Will and Mercy that you were able to resist going. Pray that your brothers and sisters are able to do the same. You can have more people in on the planning for a Muslim youth award' ceremony at the end of the year, for instance. Do this even after someone has gone to the Prom. Often times, people realize something is wrong after the fact.