This topic has an endless possibility of answers. How hard is it to have a high GPA as a science major? Well...
-- Which science?
-- What university?
-- What type of student are you?
The various answers to those questions determine how successful one will be as a science major. Are you genuinely interested in science or are you choosing a science major for the purpose of going into dentistry? Make sure it is something you want to focus all of your academic energy on, because in the end dental schools would rather see you do well in a major that is important to you as well as the necessary prereqs instead of struggling through a science major to get into dental school. I began undergrad at a large public university as a bio major wanting to go to dental school. However, when I took a Spanish course as a core class, I realized how much I missed it from high school and how important studying abroad was for me. So I changed my whole academic plan to major in Spanish, spend a semester in Spain (which I would have never been able to do as a bio major), and still take the necessary prereqs for dental school (and more!). Now I'm proficient in Spanish AND fulfilling my dream of having a career in dentistry. Lots of people are confused about the path that I've chosen academically, but in the end it is YOUR money and YOUR education. Make sure it is worth YOUR while. Good luck.