People who are nosey and fake

So my work group for my college has these 3 girls in it. At first they were nice and we were friends but then I realized how fake/mean and how much they lie. Well they are fake because we have breaks and they only text me to ask if my bf and I have had sex. They made fun of me and called me really inappropriate names. They hurt my feelings and have said stuff in front of my bf which I don't appreciate. 1. I'm a private person so regardless wouldn't tell them 2. They aren't real friends so why do I care. I am stuck being with them until January and wondering should I lie and say yes and say it was last year 
(even though they want to know immediately probably so they could mock me more. I am stuck and have to talk them and am civil but that's it. I want to lie to them to get them to shut up. Is this wrong? I don't care to have them in my life since I don't understand why they had to be mean to me. What would you do knowing you had to keep dealing with this for 5 months? They act fake nice to check in and see but it's biweekly sometimes even more. How do I handle this?