Your stomach sinks, you have that sinking feeling in the pit of your stomach, when you get a bad grade on a test.

当你考试考砸的时候,你的胃会垂下去,或者说,会有胃部下垂的感觉

耶鲁公开课 - 死亡课程节选

My teacher won't accept late work and I can't get a bad grade.

我的老师逾期就不再收作业了，而且我可不能得一个太差的成绩。

I'd rather 实战 - SpeakingMax英语口语达人