▲是的，你说的没错，在 devote…to…中，to 为介词，若后接动词，要用动名词。如：

She devoted herself to helping the poor. 她献身于帮助穷人。

They devote each weekend to cultivating their gardens. 他们把每个周末用来从事园艺。

▲但是，在你的句子中，study 是名词，不是动词，所以句子没有问题。

类似的用法如：

The students are devoted to their studies. 学生们专心于学习。

They devoted all their energies to the job. 他们把自己的全部精力投入这项工作。

He devoted his life to the promotion of world peace. 他一生致力于促进世界和平。

The book devotes a whole chapter to the subject. 该书有一整章讨论这个主题。

He devoted himself entirely to music. 他把自己完全奉献给音乐。

After he graduated he continued to devote himself to research. 毕业之后他继续致力于研究工作。

▲你的句了分析：

主句：He also told us

宾语从句：that the best way to earn respect was to devote ourselves to study and achieve high grades.

在宾语从句中：

the best way 是主语；

to earn respect 是修饰主语的定语；

to devote ourselves to study and achieve high grades 是两个并列的不定式作表语。