I've consistently gotten 35-36s on practice tests, but there were some distractions at my test center, and I missed a few questions on the Science section. As a result, I ended up with a 33 composite score. I know a 33 isn't a bad score, but I don't want to disadvantage myself unnecessarily if I can do better. I've only taken the test this one time so far. Should I retake if I'm aiming for the most competitive Ivies (HYP)?
I would recommend retaking the test. The average ACT scores at Harvard, Yale, and Princeton are 34, 33, and 33 respectively. This means your current score won't necessarily damage your chances, but it also do much to help you in the admissions process.
Our advice on the blog is if your official test score is 100 SAT points or 2 ACT points lower than your consistent practice test score, you should retake the test. Since you've only taken the ACT once, and you know that you have the potential to score at least 2 points higher overall based on your practice test scores, you have nothing to lose in taking it again.
If you are easily distracted by ambient noise, I'd also think about getting some earplugs to wear on test day. Take precautionary measures to avoid a repeat of the first test and make this one count.