How to tell someone and Who to tell ?

Okay at my school, we have this new teacher * Mr. James * Alot of girls in the sixth form are starting to get scared of him because of some things he's done. Like for example at my school the sixth form doesn' have to wear uniforms, so one day these 2 girls wore kinda low cut shirts so while we were in cl he " drops " his pen and asked the girl to get it for him he was totally looking down her shirt. Another time a friend of mine told me that he asked her to stay to help her on a quiz, and he put his hand on her knee. Then in cl we were studying French and he asked all the guys to leave the clroom to watch a movie like he let us do the previous day. So anyway at the end of cl we did a word of the day, so he wrote a word in french on the board and asked us if we knew what it meant, none of us did so he said it meant pedophile. He smiled and winked at us and was like just kidding..That's not even the worst part one day me and my mates were all in the sixth form common room and he came in there and started talking to us then he sat down and was like I'm one lucky man to be able to teach some young y girls and then he went on to say I swear if you all weren't minors... Then he got up smiled and left. So he smiled and left. That scared the hell out of us. We really want to tell our headmaster but were scared he won't belive us. Because most of us are 16 and 17. Plus he's also a really good teacher. Another thing is hes pretty young like about 28. He's always like you know you can come talk to me alone if you have a problem because I want to be their for you.. But none of the girls want to be alone with him.. So what should we do. Yesterday after cl he asked me and my mate Courtney to stay back. We didn't want to but he said it was important he wanted to go over a test with us. So after we go over the test he asked if he could have a hug, we told me no but he said it would really brighten his day.. So he gave us a hug and after that I saw him give Court a small pat on the . After that we asked all the girls to meet us in the common room to discuss what to do. We all want to do something but were scared too. What if's just joking or just feeling around and we get him in trouble. Because he said he wouldnt do anything to hurt us. Today in cl my mate Jennifer and I were