How to tell a guy you dont like him with out actually telling him?

Ok so heres the situation im in I am a freshman in highschool and I have been in school for about 2 months and I have a boy friend that I have been going out with for 2 weeks now and now theres this other guy thats in my science class he sits like 5-7 feet away from me in class we have asigned seats so the guy in science told me today that he likes me I was shocked and was nervous I have anxiety so I got out as soon as class ended I got out of class as fast as I could how do I tell him I dont like him without telling him I would have a realy hard time with that I most likely wouldint sleep for days out of guilt if I told him staight out at first I thought he was a nice guy and I thought he would be a good freind so I gave him my cell phone number now I think thats a big mistake I feel so bad about this how do I hint to him without having to tell him? I am the kind of person that has a hard time telling somwon no but hurting somones feelings is realy not me should I just ignore him that might be the easyest for me I could also tell him I have a boyfriend but I dont know if he will accept that we haveint realy talked that much maby twice?? What do you guys think? Thanks for all the help!