Islam is a religion which designs every aspects of our life. It teaches us what is lawful and what is unlawful. The scholars worked very hard to deduce rulings from the Qur’an and Sunnah with regards to our lives/ The Primary sources of Islam are the Qur’an and Sunnah and the secondary ones are ijma (general consensus), qiyas (analogy), istihsan (juristic preference)… and urf (customs). The last one is related to people’s customs, habits, cultures and relations. If these are not contradicting essential principles of Islam then there is no reason to reject them, however if some of them are not acceptable when examine them according to Islamic sources then we should avoid them. Now, your question regarding prom needs to be answered according to the criteria I mentioned above.
The prom itself is not problem but the activities, mutual relations, food etc. may be un-Islamic thus shall be rejected. It is difficult to say definite ruling for your prom unless knowing every aspect of it. Ask your heart, it tells the truth.
