根據資料，不能單獨用一個簡單的'單詞描述，而用一個句子
學妹可以說She shares the same major as me but is junior to me
學弟可以說He's in my major but a year below/behind me.
以下是關於師兄師姐，師弟師妹的一些英語資料，可以閱讀一下！！！也許能幫到你！！！
How to say "師兄"
During a delightful meeting with some readers last week at Beijing Institute of Technology, a student from Renmin University of China asked: "How to say '師兄/師姐' in English?"
That's a good question.
"師兄/師姐", or "師弟/師妹"，is from Chinese martial arts culture. Fellows learning from the same master (師父) are "senior/junior male/female fellow apprentices". As the saying goes, "The master teaches the trade, but the apprentice's skill is self-made" (師父領進門，修行在個人).
On campus these words refer to students who're studying in or have graduated from the same major at the same university. "Alumnus" (校友) may be the closest word in English. But a sentence like "John and I are both alumni of Ohio University" indicates both John and the speaker have graduated.
So, to translate "師兄師姐" into English, we have to describe instead of using single words.
Several suggestions are:
* "We're both majoring in business at Ohio University." (我們是在俄亥俄大學學商務的同門。)
* "She shares the same major as me but is senior/junior to me." (她是跟我同專業的師姐/師妹。)
* "He's in my same major but a year ahead." (他是跟我同專業的上屆師兄。)
* "He's in my major but a year below/behind me." (他是跟我同專業的下屆師弟。)