Is "Fellow" informal? If yes, what's a formal equivalent?

In this article I read that:

Fellow – Avoid using "fellow" when you mean "a person." Calling someone a fellow is more formal than calling him or her a dude, but "fellow" is still a colloquialism.

This question ensures that "fellow student" is correct, but I need to know whether it is formal usage or not.

I am writing a formal application and want to refer to a fellow university student.

Should I use "fellow student" or do I have a better choice?

As per ODO, only one of the definitions of fellow is considered informal:

(informal) a man or boy:

he was an extremely obliging fellow

Other uses are fine:

he was learning with a rapidity unique among his fellows

a fellow of the Geological Society

they urged the troops not to fire on their fellow citizens