Question: On some applications, when you have to check off what "school" within a university you are applying to (i.e. College of Arts and Sciences), what do you do if your major is undecided? How do you know what classes to take once you're in college?
That's a good question, and there are no easy answers. If you are applying to a liberal arts college, then usually the application will ask only what you think you want to major in and you won't be bound by the commitment. While it's fine to write "Undecided" (and many students do), we suggest instead that you give admission folks a sampling of your interests--and maybe of your personality, too. For instance, you might say, "As an avid Jane Austen fan, English tempts me, but I'm not ready to give up my passion for science yet either." (Electronic forms might not give you the leeway to write all of this, and you may just get stuck with "Undecided.") Our other piece of advice is that, if you're wavering between a very popular choice (e.g., psychology) and a less common one (classics, Italian), then go with the road not taken. Remember, it's not binding.
However, when applying to some colleges and many universities, you must indicate what "school" (or sometimes what specific major) you want, and your decision may be important and hard to wiggle out of. Most universities have a "School of Arts and Sciences," as you've mentioned, which is basically like a liberal arts college within the larger university, and it's often the choice of uncommitted candidates like yourself.
If, however, another school ("Allied Health Science," "Architecture and Planning," etc.) intrigues you, then you need to ask admission officials how easy it is to make a switch, if you get there and realize you haven't made the right pick. At some places, it's not a big deal, while at others there may be no room for you left in the department you decide on or in many of the classes you need. You may end up having to either opt for a second or third choice of major, taking extra semesters (or years) before you graduate, or transferring to another institution altogether.
That's why liberal arts colleges are good bets for students with little clue about academic goals. They offer you a chance to experiment with a range of subjects, and you're likely to find something you like that perhaps you've never previously encountered. Once you get to college, you will probably have a "core curriculum" or "distribution requirements" to fulfill, and these will give you some direction. You will also work with a faculty adviser who can give you suggestions, too. Some colleges offer a mix of liberal arts and pre-professional programs, but you don't have to commit to an area of concentration until the end of your sophomore year. These can be a good bet for undecided high school students.
Here's an exercise that may be worth trying: get your hands on one of the huge college megatomes such as Barron's Profiles of American Colleges. In one of the appendices, you'll find a list of a gazillion different majors--from predictable ones like history and political science to less familiar fare such as "Labor Studies," "Toy Design," and "Air Traffic Control." Although reading through this list may not tell you exactly what you want to do with the rest of your life, it could open your eyes to options that you didn't even know were out there, and you might decide to direct your college search towards institutions that offer some of the fields that most intrigue you.