College takes work. The courses are more challenging than high school classes and you are responsible for your own learning. Your professors won't hold your hand; it is your responsibility to earn your grades. When you party too much, you sacrifice study time necessary to do well on tests and assignments. You're also more likely to skip classes when hungover, missing valuable information.
Skipping classes and poor performance on assignments and exams yields a poor academic impression. Your teachers are responsible for participation grades, based in part on attendance. These grades can mean the difference between letter grades. Also, you may need reference letters from teachers for jobs, internships or scholarships. If performed poorly and never attended, they may have a poor impression of you and will write your reference accordingly.
When partying, peer pressure and alcohol often leads to poor decisions. The the Canadian Institutes of Health Research (CIHR) Institute of Neurosciences, Mental Health and Addiction found peer pressure to be the most significant influence on university binge drinking. Everyone understands a few debaucheries, but if you repeatedly get very drunk and make a fool of yourself, people notice. Rumors swirl and people grow tired of your partying while they study. You may lose friends or have a reputation following you in the future.
Even when you're young, partying affects your health. Late nights and short sleeps can wear you down and lower your immune system. Drinking also lowers your immune system. Too much partying makes you susceptible to illness, both minor and serious. According to the Numours Foundation, short term effects of drinking include lowered intelligence and poor sleep. Neither of these consequences bode well for your grades or overall health.