学姐咋说

直接叫:sister

e.g.
My dear brothers and sisters, you have finished your studies in elementary school.
亲爱的学长、学姐，你们已经完成小学阶段的学业。

洋人好像没有这种说法吧...

senior

elder senior

senior 1.[美]（大学或中学的）毕业年级.
2.(大学)四年级学生,(中学)高年级学生.
sister (不肯定).
抱歉了,没查到.