I moved to the US when I was 7 years of age. Although I speak Spanish fluently I heavily struggle in grammar, so I decided to join Spanish 5 honors my freshman year to improve it (all other freshmen take either spamish 3 or 3 honors). The content in the class is advanced enough to challenge me in every way possible, therefore heavy studying is still done by my part for every quiz and test. Since there is no way to explain this to a college when applying, will they think this is a bad thing? Although surely not the case, I am afraid that colleges will think I took the ‘easy’ way by studying Spanish and not another language.
@Aphromo - That is a good question and one you might put your guidance counselors. I do know that SAT subject tests taken by native speakers are not much valued by colleges. Test scores actually distinguish somehow between native and non-native speakers. I don’t think it will hurt you, but it won’t be much help either.
No. Heritage speakers fall everywhere along the proficiency spectrum. Some can read/write/speak the language. Others can speak it, but with many grammatical errors. If having parents that speak the language was all that was needed to become proficient, every native English speaker in the US would not need to take English every year in elementary and high school. There are many things that one might worry about in the admissions process; this is not one of them.
Don’t worry about it. (You would check ‘spoken at home/understand’ and not check 'read’or 'write’on the common app.)
However you may want to start a real foreign language next year depending on what colleges you’re aiming for.)
I will assume @MYOS1634 meant “another foreign language” not “real foreign language.” 
That’s part of why many schools classify them as “world languages” or “languages other than English.” I would agree that you might want to look at another language if there are no Spanish courses past Spanish 5. If there is a Spanish 6, I would take that, as I am a proponent getting as high a proficiency as possible before trying another language.
Well, I meant 'a world language that is really foreign to you ’ vs. ‘a world language that is also a heritage language for you’.
I agree that if your school offers higher levels (AP Spanish literature for example) it’s valuable to reach the highest possible level.
Take it and do good. Raise your GPA easily and get a good SAT 2 score.