I'm going to say no to a girl asking me to prom at another school
Do you not want to go with her to prom?
Yes I'm going to reject her, she's the most ugly human being on earth
Even though your reasons are mean, it is okay to reject an invitation. You don't have to accept everybody who asks on a date and it will be seen as okay by others as long as you are not mean to her.
Yeah, I just really don't want to go with her.
Well as long as you let her down gently then it's fine to deny her from taking her to prom.