Going to parties, drinking alcohol, and wasting time is a form of learning. How to live life, how to make mistakes, and where you're going to fit in in the world is probably one of the most important thing you learn as a teenager.
If a kid learns to have just one drink, don't be a junkie, use protection and waste time only judiciously by 18, they will be able to learn more from higher education or career than if they learned it's good to act like a mindlessly hedonistic animal with no concept of the future.
You can spend your first 40 years sticking your tongue into electrical sockets, this is technically some form of learning, but it's not a great way to have a good chance in life. There is all the time in the world to waste and act stupid, after you have the basic bearings of adulthood.
Teenage impulsivity and risk-tasking are reproductively optimal strategies (the actual rates of death and disability from these activities are usually low anyway) even if you like to look down your nose at them. And people who engage in those activities often end up doing just fine in later life, as well-adjusted, popular, confident adults, with a history of sexual success.
Maybe they're not optimal strategies for being a high-performing time-efficient adult, but so what? Natural selection only cares about babes and babies.