Principal won't let me go to my boyfriend's homecoming.?

My boyfriend and I have been dating since I was in highschool. I graduated in 2009 and we've been dating since 2008. He is a year younger then me but 2 grades lower. He is currently a senior in highschool and his homecoming is coming up soon and I want to be able to go with him.I wasn't able to go to last years homecoming or junior prom with him because the principal of the school decided THAT year that if you didn't go to that specific school currently then you were not allowed to go to the dances. So that means if you graduated already or even if you are still in highschool but went to a different school you still weren't allowed to go. Out of my four years in highschool we never had this rule until last year. I think this is ridiculous, what should i do? I was thinking about e-mailing the principal but i tried that last year for junior prom & I even had a parent call and it did absolutely nothing. What are my next steps? I am not looking to just go reminisce at my old highschool.. I want to share these memories with my boyfriend and be apart of his things like he was at mine. Plus I dont think its fair what so ever that every other highschool in the school's district is allowed to bring an outside guest but this particular school is not?.. Thank you every comment is greatly appriciated.