Guy issues!...i need advice!?

so im a junior in high school and there’s this guy that I have four cles with: science, math, language, and English. Basically he sits near me in all four cles as well. Ive known him since the third grade and basically he has changed so much since I first met him (he’s gotten a lot more attractive?) so he has this tendency of making it come off to teachers that we’re flirting. The problem is that he basically has a gf but there not official but he’s told all his friends even me that he really likes her and she likes him back. So in all my cles he’ll make fun of me like he’ll do things on purpose to bother me and he’ll point out the slightest details of what I do and he overyzes everything I do….like today I was sitting down and then he leaned into me and then like sat on my lap?...and then I was like “omg wat r u doing get off!” and he just started laughing and was being all loud and said stuff like “nat!...i know u want meto be all close to you like this but geez I may have to tell my girl!” so im confused cuz even when he bothers me and/or is mean to me….he does have his sweet moments like he’ll help me in the subjects that im not good in and help me improve and also when he says mean or annoying things and he can tell that I thought that he crossed the line and he’ll look at me seriously and take my hand or put his arm around me and say he didn’t mean it and that he’s just kidding blah blah blah…and he says he does it just to mess with me….so can u guys give me advice on why we have such a weird relationship…and what should I do when he does this?....and why do you think he does this? O_o