Obviously different locales will have different RULES, but regardless of the rules I wouldn't be caught in any sort of semiprofessional sort of situation unless I was in a.) khakis and buttondown shirt or b.) scrubs (if it's common in that place's culture). You want to fit in with the professionals there, not stand out as not belonging. You may still be pre-med, but you're asking to be allowed to be a part of the professional environment, so you have a responsibility to do whatever is needed to maintain a professional environment for the patients.
That said, I've started to slack off on wearing ties, even in our clinical classes. They're just germ spreaders, anyway. I don't think anyone has even noticed, and it still looks plenty professional, especially with the lab coat over it. However, if anyone were to say something to me about it, I've got one rolled up in my gear bag that I could throw on. You hafta fit in with the culture 🙂