grade, grayed

Grade or grade means level or stage--
She is in the 5th grade.
He will be in a higher pay grade next year.

Grade also means a measure of quality, physical or intellectual--
This is a higher grade of diamond, so it's more expansive.
She got a good grade on her last exam.

Grade also means to decide on a level of quality, or to judge the level of quality--
The teacher will grade our papers this week.
He will grade the papers by tomorrow.

Grade also means the level of a road or trail, or to make a road or trail level.--
This trail still has a steep grade.
The road crew will grade this until it is nearly level.

Grayed or grayed ( rhymes with grade ) means turned gray, as of hair or unpainted wood--
By the time he turned 50, his hair had grayed.
The unpainted house grayed with the years.

Now that you know that, you can say--
"His mother was sure that worry over his failing grade was the reason her hair had grayed so quickly."

Note--"grey"  is either British, or it was written a long time ago.