A few years after he graduated,

[Bassim]

Have I made any mistakes? Is it correct to say "student days"?

A few years after he graduated, Ben lost contact with almost all of his fellow students, although they had promised to one another to meet once a year to refresh the memories from their student days.

 

[GoesStation]

"Their student days" is fine. However, earlier in the text you've used a preposition that doesn't belong there.

 

[Bassim]

The preposition "to" should be omitted.
"Promised one another" is the correct phrase.

 

[Tarheel]

I prefer promised each other.

 

[abaka]

You may also omit the "each other" altogether.

 

[Tarheel]

You may also omit the "each other" altogether.

Yes indeed!