Is letting hair grow long really a Sunnah for men?

Salam alaykum, everyone. I'm studying at an Islamic school, and we have a rule that guys must keep their hair short. One teacher told me that having long hair isn't actually Sunnah. His reasoning was like: just because the Prophet (peace be upon him) drank water doesn’t mean drinking water is Sunnah, so same thing with hair. But honestly, I'm confused. Can someone explain if growing long hair is truly a Sunnah or not? Jazakallah khair.