根据资料，不能单独用一个简单的单词描述，而用一个句子
学妹可以说She shares the same major as me but is junior to me
学弟可以说He's in my major but a year below/behind me.
以下是关于师兄师姐，师弟师妹的一些英语资料，可以阅读一下！！！也许能帮到你！！！
How to say "师兄"
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 '师兄/师姐' in English?"
That's a good question.
"师兄/师姐", or "师弟/师妹"，is from Chinese martial arts culture. Fellows learning from the same master (师父) are "senior/junior male/female fellow apprentices". As the saying goes, "The master teaches the trade, but the apprentice's skill is self-made" (师父领进门，修行在个人).
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 "师兄师姐" into English, we have to describe instead of using single words.
Several suggestions are:
* "We're both majoring in business at Ohio University." (我们是在俄亥俄大学学商务的同门。)
* "She shares the same major as me but is senior/junior to me." (她是跟我同专业的师姐/师妹。)
* "He's in my same major but a year ahead." (他是跟我同专业的上届师兄。)
* "He's in my major but a year below/behind me." (他是跟我同专业的下届师弟。)
参考资料：http://bbs.lwoods.net/dispbbs.asp?boardid=52&id=357545