"He is an excellent student."
Translation:Αυτός είναι ένας άριστος μαθητής.
As far as I understand it, it's much the same as in English. τέλειος is "perfect" and "άριστος" is excellent. Something can be excellent without being perfect. But if something's perfect/τέλειος, then it's without flaws or drawbacks. eg "I scored 96% on the test, an excellent result, but not perfect". ("Το σκορ μου στο τεστ ήταν 96%, άριστο αποτέλεσμα, αλλά όχι τέλειο").