Underage drinking in and of itself may be victimless in some sense, but schools want students to be responsible, and from a liability point of view they can't simply ignore it when they see it happening. But neither do they want to refer all such cases to law enforcement. So therein lies the desire for education programs and addiction counseling when it is indicated.
Sexual assault is much more complicated. If the accuser doesn't want to report it to law enforcement and press charges, the maximum penalty the school can inflict is expulsion. Historically this has been a problem in a variety of ways. First, schools have sometimes been accused of discouraging the accuser from going to the police. That problem has resulted in dedicated sexual assault response plans to avoid that sort of thing and work with the accuser to connect them to appropriate resources, counseling, etc. Beyond that, the school is in a bind: prior to either legal action or a school-based disciplinary hearing for violation of the code of conduct, it is punitive to inflict punishment like suspension or force schedule changes to avoid the accused coming into contact with their alleged victim. At the same time, if there was an assault, it can very much exacerbate the harm to the victim to keep things as they are. I don't know of a good solution to this. It often results in the accuser being the one that has to change their schedule or drop out to avoid coming into contact with their alleged attacker.
Again, I don't have an answer that both completely protects a victim from further trauma at the same time that it provides the accused with protection from penalty until they are found guilty. Personally, I think it helps the situation when the accuser is willing to go to the police because that opens up other options like a restraining order that can both protect the victim but also provides judicial review before the accused's rights and privileges are abridged. That is still far from ideal in many cases though. No easy answers here.