"优秀学生干部" 用英语可以说成 "outstanding student leader"。

"outstanding" 意思是 "优秀的、杰出的"。
"student" 意思是 "学生"。
"leader" 意思是 "领导、干部"。

例句1:

He is an outstanding student leader in our school.
中文翻译：他是我们学校中一位优秀的学生干部。

例句2:

She was elected as the outstanding student leader of the class.
中文翻译：她被选为班里优秀的学生干部。

例句3:

The outstanding student leader organized a charity event to raise money for the poor.
中文翻译：优秀的学生干部组织了一场慈善活动，为穷人筹集资金。