I second @Rarebear.
Yes, you are bilingual according to your description of your linguistic behavior. However, do you have a native-like command of Spanish? No. With your present linguistic competence in Spanish, could you follow academic studies in Spanish? No. Will it be hard for you to get the basic competences to carry on studies in Spanish? No. It will be easier for you than for students who don’t have the background that you have in Spanish.
I think that the question of your college application is for statistical purposes and you will not be lying by stating that you are bilingual because you are not claiming you have a native-like command of Spanish.
Also, keep in mind that most people do not have the same level of competence in all four skills, be it your native language or a foreign language. So if you can speak but can’t write Spanish, you are bilingual. If you can’t read that well but can understand a conversation, you are bilingual.
I think you need to clarify your concept of what it means to be bilingual. For that, you need to read a bit on the subject.
The term itself is very difficult to define, even though at first glance it seems so simple: bi-lingual = use of two languages. However, the term has not only linguistic implications, but also political, social, cognitive, psychological, etc. There are different forms of bilingualism and usually one language tends to become the dominant language with the other in the subordinate role.
The popular view would tell you that a bilingual person speaks two languages perfectly, and in fact some authors share that opinion and define the concept of a perfect bilingual as one who has native-like control of two languages. Others, however, indicate that it implies the minimal competence in one of the four language skills (speaking, listening, reading, and writing) in a language other than his mother tongue, and there is a whole array of definitions in between
What you are specifically referring to falls in the category of Bilinguality or Individual bilingualism which is the psychological state of the individual that uses more than one language where the degree or level of proficiency will vary along a number of dimensions, namely emotional and cognitive.
In educational/academic terms (specific purposes for the language) you need to make sure that your linguistic structure and language skills are sufficient so that you can meet the communicative demands of academic settings.