Not to throw cold water on your plan, but one thing comes to mind. If I correctly understand your situation, you are in high school and have not studied much Spanish. You are also a planner. You might think that choosing a career now and zealously pursuing it is a good idea. However, many people begin college with one idea and end up changing completely. It's neither good nor bad, just the result of having new experiences and growing up. If you begin a 4-year program at a Spanish university and decide you don't really want a degree in Spanish, your options may be limited. Or you may feel stuck and will never feel the freedom to explore and discover what you really want to do. That is what college is all about. Being too sure of your desires at this early stage may serve to close your mind to fabulous opportunities. Like I said, I don't mean to be a naysayer. It might work out great, but having been there done that, I just wanted to add a different perspective.
On the other hand, at a university in your home country, you can always change course with little repercussion. If you stick with a Spanish major, you can spend a year in Spain and your fluency will really take off. I suggest spending some time in the summer at language schools to rapidly enhance your Spanish. Just make sure your university will give you credits for it. Also, you might want to try a homestay with a Spanish family. I've done both and they were some of the best experiences in my life. Like DCS, I heartily recommend spending your time with Spanish speakers and avoid speaking English. (A Spanish sweetheart wouldn't hurt, either!)
Good luck!