Most Helpful Guy
It's gonna be cringey if you wear cat ears at school1
Most Helpful Girl
I've found kitty cat ears at Hot Topic, they tend to have a lot of different kinds.
As for clothing, you have to think school appropriate first and foremost. Ariana wears a lot of crop tops with skater skirts and heels. Crop tops may not be appropriate for school (unfortunately!), so maybe opt for a fitted tank top and t-shirts instead and then go for flared skirts. She also wears knee high boots and socks with heels sometimes too. So you could try finding some of those.
Ariana also wears a lot of baggy shirts and sweaters over leggings or tights. You could try and find some stuff like that. I would search all over. Skinny jeans with a tank top and heels would also work.
I feel like Ariana would also wear one of those mini back packs. Imagine that with a big baggy sweater and a cute flared skirt (I think that would be something Ariana would totally rock).
And don't forget her signature high pony! You can do a slicked back high pony (maybe buy some hair mousse to keep it slicked back). Or you could do a half updo, take half your hair, slick it back into a high pony and play around with it so that it falls making it look like you have really long hair. It's super cute and kind of Ariana-esque.