I just wrote a long post explaining why I think you should take a class as opposed to studying on your own, but I accidentally closed it before I submitted it. I'll abridge. No bones about it: Latin is tough, and real Latin (as in, Cicero, Horace, Tacitus - your basic primary authors that will probably appear on a language exam) bears only a slight resemblence to the exercises and sentences you'll find in a grammar book. They're clever; they play with words and structure. You have to recognize the nuances. Latin grammar seems very straightforward, even overly specific, at first glance, but each case can be used in many different ways and each word can have many different, sometimes even contradictory meanings. And you don't get any prepositions, regular word order, or long marks to help you along the way. I took Latin every semester for four years with excellent professors, and it still takes me a good 30 minutes to an hour to translate a page of Latin, depending on its difficulty. Dead languages are not something to be undertaken lightly. To acquire real proficiency, I think that coursework is necessary. UNLESS, however, you can somehow take a medieval Latin exam rather than classical. With a good medieval glossary and a standard Latin grammar book, I think you'll be able to get by since it's much closer to modern Romance languages in word order and usage (they actually overuse prepositions!), and also you won't be put off or befuddled by the changes in grammar and such from classical Latin.
So that's my two cents, but probably the best person to ask will be one of your profs. They'll know the level of difficulty of the texts on your school's language exam, and thus how much preparation you'll need.