"To improve one's grades" 或 "to achieve better grades" 都是常用的表达方式。

"To improve one's grades" 是从英文 "improve" 和 "grades" 两个单词构成，"improve" 意为"改进，提高"，"grades" 意为"成绩"。因此，"to improve one's grades" 就是指"提高成绩"。

"To achieve better grades" 是从英文 "achieve" 和 "better grades" 两个单词构成，"achieve" 意为"实现，达到"，"better" 意为"更好的"，"grades" 同上意为"成绩"。因此，"to achieve better grades" 就是指"达到更好的成绩"。

下面是一些例句：

I need to study harder if I want to improve my grades. (如果我想提高我的成绩，我需要更加努力地学习。)
She has been working very hard to achieve better grades in her exams. (她一直在努力工作，以期在考试中取得更好的成绩。)
He is determined to improve his grades this semester. (他决心在这个学期提高他的成绩。)
They have been putting a lot of effort into achieving better grades. (他们一直在努力达到更好的成绩。)