honorifics that would be used by high school-aged students who are the same age
If the same-aged person has a title, you can use it as an honorific
If 陳同學 is the 學生會長 of your school, you can call him "陳會長"
If 張同學 is your sports team's captain, you can call him "張隊長"
先生 (Mr.) is the universal honorific for an adult male of all ages, 陳先生 can be your age or ten years older
女仕 (Ms.) is the universal honorific for an adult female of all ages, 張女仕 can be your age or ten years older
小姐 (Miss) is the universal honorific for a single adult female
To show respect, politeness, or fondness, same-aged people would add 兄 before the last name or first name of a similar-aged friend
陳志豪 --> 陳兄 or 志豪兄
張國強 --> 張兄 or 國強兄
If you know for sure that you are younger, you would use 陳兄 or 陳老兄
If you know for sure that you are older, you would use 張兄弟 or 張老弟 instead
A polite address between same-aged people is not considered honorific.
Generally, high school students would address their fellow students by their full names only, using honorific or polite addresses is considered unusual and too formal
Using nicknames might be the most common way to address same-aged high school friends