Advice needed ASAP

So I like this guy, he’s in 4 of my lessons. Sometimes I feel like he likes me back by the way he acts towards me and then sometimes I don’t. We flirt quite a bit when we are together and twice he has come up behind me in lesson and tickled my sides and touched my bum (I don’t mind personally). Yesterday I posted on ig and he comment “omg no tag I’m disappointed 😢😢 jk”. This made me get that warm fuzzy feeling inside, he never comments on my posts. Ofc I edited it and tagged him. A few of my friends know I like him and they tell me to talk to him as we don’t currently talk outside of school, but I have like 0 confidence when it comes to talking to boys over Snapchat or Instagram. Wtf do I do?