I'm going to call my classmate unambitious
Has your classmate been criticizing you as well?
Yes, he told me I'm unambitious and that I'll never improve any further.
While it's understandable to be upset at having your character slandered, it isn't appropriate to go and do the same thing to the person doing it. Two wrongs do no make a right and it's always rude to denigrate other people. Rather than call your classmate names, try to focus on yourself and prove what he said is wrong by succeeding in your own pursuits.
I'll think about it focusing on myself.
I am glad that you will think about focusing on yourself instead of denigrating your classmate. I know it can be upsetting to be called names but responding in kind will just escalate things.