(*dons the hat of a devil's advocate*)
I wear it at school, and people are always either giggling and saying something nice or just completely laughing behind my back....
Now I wear it for a statement. There is nothing in the rules that says I can't wear a tail. It doesn't endanger anyone, and it doesn't interfere with the education, neither. I'm fighting this!
If this "giggling" and "laughing" you refer to occurs in classrooms, then your dean might have a point. A tail has an atypically high potential to be an attention magnet, and an attention magnet in a classroom detracts from the attention of other students who would otherwise be paying attention to the lecture material.
In such a case, wearing a tail in class is much like playing classical music on speakers in class. It may help the one engaging in the activity, but for other people, it is distracting. The one who is able to focus more while listening to classical music should instead listen to his/her music using headphones/earphones (after explaining to the instructor the purpose of the music) so that no other students are not distracted by the music. In a similar sense, it may be appropriate for you to find something less distracting to other classmates that comforts you like your tail does. Furry shirts, a collar, and/or a collar bracelet might qualify, but you might come up with other similar ideas.
Of course, what I have noted is only relevant in the context of a classroom; wearing a tail elsewhere on campus should not significantly interfere with anyone's education, and I don't think arguing this civilly (and I emphasize the "civilly" part; defensiveness neither sways hearts nor fosters good relations with faculty) should prove difficult. Additionally, it's possible that your classmates are not significantly distracted by your tail (or, more precisely, they are not significantly more distracted by your tail than they would be if you wore particularly mundane attire), in which case the argued point is less relevant to the degree that your classmates are not distracted by your tail.
Regardless, I hope you find a favorable resolution to your present issue with wearing a tail on campus.