If it wasn't so mean-spirited, it would be funny...

[Ser Clegane]

she will wreck everything in her way to fulfill her selfish needs

And again: How did she wreck anything?

The school decided to cancel the prom. She did not present an ultimatum along the lines of "Either you will allow me to come with my date or I will make sure that nobody can go to the prom". This was solely the decision of people who rather cancel the fun for everybody instead of allowing a person something that could not possibly have ruined the prom for anybody who would not have chosen to have it ruined.

Your line of argument is in no way different from any argument that could have been made in the past for segregation.

you don't win arguments by forcing your view on others

This is also a very strange argument in this context

Firstly: What did she try to force on others? The simple presence of a gay couple? It seems to me that only the school is guilty of forcing a view on somebody. In this case for her the effects (not being allowed to come to the prom with her date) are much more tangible then the potential effect on others (having to "endure" the mere presence of an openly gay couple - and I have not even read of another student commenting that the gay couple would have ruined the prom if they would have been allowed to come)

Secondly: What would have been the alternative to "win" the argument? According to the arguments so far, just being openly gay already qualifies as "forcing your view on others".
Should she have simply done nothing? Not coming to the prom at all? Sitting at home and cry? Guess that would have really helped to "win" the argument and to convince the school that the next year they should be more tolerant ...