He has a girlfriend and he is flirting with me big time!?! HELP Im Clueless!!!!!!?

We met at the Vatican (italy) on a school trip. Through out the whole two weeks he was totally flirting with me by holding my hand, puting his arms around me and saying he loved me, always touching me, and making gestures when he would walk by me like saying "mmm". People on the trip told him in front of everyone to stop flirting with me because he has a girlfriend. A month after the trip i saw him at the gym and he slapped my . I said "that really hurt" and he said "dont worry ill make it better". People told me that is not how he acts with other girls. He is pretty popular and if his girlfriend wanted too, she could make my school life hell! I wouldn't say i flirt back with him. But this year im going to be in to clubs with him and a cl with him! What do I do!? Im so clueless!