I often get asked this question, especially at the beginning of the year.
We recognize that, no matter the language, there are differences based on several factors, for example, regional accents, phrases and slang. However, the grammar (the structure) of the language does not change. Think of the English spoken in the United States and that spoken in Great Britain or Australia; despite the accents and some different words, the grammar is the same.
The same with Spanish. The grammar between Argentina, Spain, Mexico and all the other Spanish speaking countries does not change. This is the Spanish that I teach in the classroom. The Spanish speaking world is large, and as such, the vocabulary that I teach in the classroom is what is readily understood throughout the Spanish speaking countries. The accent taught is also the most universally understood; that being said, as much as I love the "castellano" of Argentina, it's not what I teach in class! Dang!