I don't know what your teachers are thinking, but I don't allow sophomores in my AP Bio class. I require that they take chem and regular bio first. AP Bio is usually very tough for sophomores (not because they're not smart, but because they are lacking in the science back ground AND they usually haven't developed their study skills yet.)
FYI...in college they say you should study 3 hours outside of class for every 1 hour you are in class. I don't necessarily agree with this, but I tell my students to study about 1 hour per night. If you aren't willing to put in the time outside of class, don't bother taking the class. Look at your schedule and activities and make sure you have enough time to not only study for your classes, but to enjoy your high school experience. The last thing you want to do is miss out on some of the awesome things that high school has to offer.