^i think its debatable. there certainly isn't a withdrawal syndrome thats comparable whats caused by long term opioid, alcohol, barbituate or benzo use. with those substances, theres an actual sickness with measurable effects (ie. vomiting, seizures, sweats etc.)
whereas with cocaine (and other stimulants), users report feeling lethargic and depressed when abstaining, but theres no measurable physical symptoms besides that. so in a sense, its all in their head. whether thats described as physical or psychological is a whole subject that can be debated (as in, what is the physical cause of our psychological state? is it a separate thing?)
to the OP: i would just drop it. i mean, most drug users would probably anecdotally agree with you, but your not gonna find anything that would hold weight for your teacher i think. with people like that, it doesn't matter what you do or say, they are not gonna change their mind on the subject. as a drug user, it's frustrating, but you're gonna run into a lot of people who don't understand drug use in your life, so get used to shrugging it off.
good luck.