So I've been trying to tell my parents that I don't think I should go to prom because I think it would go against my morals and be an occasion of sin.  They said they don't want me to live to regret not going.  I still don't want to go but what is a valid argument I could give against my parent's "concern".  

Nothing wrong with dancing traditional dances and normal music with other trads. Other than that though, I would definitely avoid it. As a young guy attending college (thankfully living at home and not in the dorm) the dress of the females once it hits 65 degrees Fahrenheit is pretty revolting. 30, 40 years ago they would have been mistaken for whores. I don't even want to think what dancing with people like that would be. Where's the barf emoticon when ya need it?

So I've been trying to tell my parents that I don't think I should go to prom because I think it would go against my morals and be an occasion of sin.  They said they don't want me to live to regret not going.  I still don't want to go but what is a valid argument I could give against my parent's "concern".  

You'd also regret not being true to yourself (your goal to be obedient to God in all things and hence, avoiding something that you know would create a lot of temptation for you).  In the end, it seems like it is about priorities.  What's more important:  marking the end of your school days and time you'll spend with most of your classmates with a dance and dinner or avoiding an evening of watching lots of your classmates fall into sin and being tempted to yourself?  

Perhaps there are other things you could do before the end of your last year in school, to say goodbye to everyone and mark the passing of this special time in your life.  

Your parents are probably looking out for your interests.  So you'd be wise to think of little ways to daily mark the passing of this time and regularly share those experiences with your parents.  Of course they may never change their minds, but that should at least ease their minds.