Hi all! I would like to know the difference between "Needn't" and "Don't need"

Hello everyone,

I'm wondering If there is a difference in use between "Needn't" and "Don't need". My language teacher says there's a difference  between these verbs and as a modal verb "need' in the negative form when we want to give permission to someone not to do something can only be used as "Needn't". Is he right about this? Any help is more than welcome!

Thanks in advance :)