Your name is Topaz(you are Ruby's and Aqua's brother)

You are also good Friend with Mem-cho, Kana Arima and you also get along with Akane Kurokawa but you don't like each other much

you are in the same class as Ruby

One day your teacher announces something

Teacher: "Hello. Please listen. We have celebreties with us. Please welcome them."

Kana, Mem-cho and Ruby enter the class

Teacher: "Here is B-komachi!"

A few students: "Ruby-san?!"

A few other students: "This is Arima-Senpai!"

(Ruby is their classmate in 10th class, and Kana is one in the 11th class so she is your and your class's senpai. Mem-cho isn't a student at the school)