What do you even learn in music colleges? I play the drums, and I want to know what I should be ready to learn (and learn some of it now).
A typical Bachelor of Music degree will consist of complete sequences in music theory, history, and ear training, regardless of your instrument or specialization. Some programs may even teach these areas all at once in a "general musicianship" sequence, but this is not very common.
These sequences will generally last for two or three years, with the remainder of your time spent in higher-level elective courses branching out from those areas at a different level or with a certain specialization, or predetermined courses specifically related to your major, be it music education, business, therapy, or performance.
Throughout this time, you will also be part of the percussion studio, and have weekly lessons with a professor or doctoral candidate, depending on your school. As a percussionist, you will become proficient on the full range of orchestral percussion instruments, including timpani and mallet instruments (marimba, etc.). You will likely audition into a large ensemble every semester, which may include a concert band, wind ensemble, or orchestra, depending on the size of your school. Also at the end of every semester, you will have some kind of performance examination (often called a Jury) in front of a small group of faculty members. Usually, your degree requirements will culminate with a recital or series of recitals.
Drumset is typically not a strong academic focus, largely because everyone already knows how to play drumset, and the other percussion instruments and general musicianship are much more important. If you are a good drumset player, you will have plenty of opportunities in student-organized bands and jazz combos. Unless you're at a school that specializes in jazz or popular music, there will typically only be a couple of opportunities to play drumset in for-credit ensembles, but of course this varies by school.
As for what you should start focusing on now, theory and mallet instruments would be my strongest suggestion. The earlier you start learning your way around a marimba, the better! You might also want to look at some audition materials for schools you are interested in, and see if there's anything on there that you will need to spend a lot of time on. Finding a private percussion instructor for all of this is also highly recommended, and they can offer more information and suggestions for what you should be working on.
Consider this answer supplementary to the main answer of what you learn academically. Classes and lessons are only part of the value of college.
Perhaps the most important thing in college or conservatory is the community itself. Music is not something you can learn in isolation, and just being a great player by itself is not going to have people calling you on the phone to hire you.
Classes and practice are important, of course, but you need ensemble and performance experience. You'll have opportunities to get together for sight-reading sessions or jam sessions. The composition students will need you to perform their pieces and the conducting students will need you as well. Everyone has their eye on outside gigs to make extra money; you can form an ad-hoc band or small ensemble to take advantage of this.
In your career you'll need a network of people to tell you where the jobs are and give you an introduction. You start building this in college.
What most music schools DON'T teach you (Berklee being one exception) is about the business of music: how to manage your career; how to manage a band; how to be a session musician (as opposed to a soloist or orchestra player). You can learn this from others already in the business.