How do you know if a girl is interested or just being friendly?

so i mean like classroom situations. what are things girls do with guys they like versus guys they're just being nice to? i'm reserved and not experienced with girls and relationships so i really don't have any clue. i'll be honest, i'm about 5'7", i have a decent face, ranging from 5-7, like i'm not super hot but i'm not ugly and i have a nice smile. thing is, all through school i was very skinny, hardly over 100 pounds. but, summer before senior year i put on about 20 pounds of mostly muscle. i'm still skinny but 20 pounds is a big difference and with my height and frame i look toned rather than anorexic. before, girls never glanced twice but now they notice my existence and people never call me twig arms anymore. i'm not popular and never have been so growing up i was very shy and awkward. now that i'm fit, eat healthy, and workout, i come off as just quiet but confident. it used to be no one knew i existed so i just focused on my work and i still do but now i'm not afraid to be myself and express it occasionally through personality, music, clothes, etc. i'm laid back and don't make an effort to socialize but i can be funny from time to time. i know this because people actually laugh. but, i don't care about making friends so people probably think i'm weird because they don't know me but i can't give less of a ****. i have always been like this but it used to be never around strangers. just to be fair to myself, looking at the whole picture, i'd say i'm somewhat of a catch. i understand i won't be scoring angel faces with huge tits and fat asses and i don't care. i just wanna girl who's pretty and nice. point is, i don't know what to make of this attention i'm getting from girls because it's completely new to me. because of how i grew up i would go crazy over any girl who said anything to me or even acknowledged that i existed. it's pathetic i know. i taught myself to ignore any of the slightest hopes to avoid pain and humiliation. but now it seems to me like girls have been giving obvious signs i should make a move but because of how i am i just ignore it and tell myself i'm being crazy again. so back to the question how can i tell if they want me to talk to them or if they're just treating me like any other guy? fear not, i'll give examples. "girl 1" i noticed her looking at me a lot during class but i always just glanced away and continued on with my work. one day i wore athletic shorts and it seemed like she was practically eye ******* me. i thought to myself that she probably just thinks i'm weird or something so i just ignored it. this one time we had to find partners for a project and she was sitting behind me. i stood up and looked around for a partner and i heard her asking someone if they wanted to work with her. i assumed she wasn't talking to me but after she asked i glanced down and she was looking right at me. i bitched out and acted like i had no idea. "girl 2" in class there was a project with assigned partners and we were put together. she's friends with girl 1 and when the partners were assigned she said something to girl 1. i didn't make it out completely but all i heard was "sorry" and we held eye contact for a good few seconds before she came over. when we were working she seemed really friendly and laughed a few times at things intended to be serious. later on in the year we had assigned seats. she was to my left and out of the corner of my eye i saw her glancing in my direction multiple times and stroking her hair a lot. everyone around us were girls. i figured it was for someone else on the other side of the room maybe and left it at that. "girl 3" you know how desks have 4 legs and horizontal support bars going around them. i had propped my feet up on them and the girl next to me put both her feet up so they were touching mine. this caught me completely off guard and confused the **** out of me so i said nothing and just froze like a deer in headlights. she didn't say anything but didn't move either. she was way out of my league and i have read that girls like to flirt with guys just for the reaction so i was like **** it i'm not gonna be a statistic. "girl 4" she constantly talked to me about random **** and would sit kinda facing me sometimes and would fix her hair and makeup a lot. in this particular class i had with her i joked around a lot and she would always laugh but i was pretty much the class clown so i knew not to get my hopes up based on that. she's pretty outgoing so i made sure not to fixate on how she talked to me a lot. all in all these are my best examples of what i'm referring to when asking this question. i'd really appreciate some insight on whether i'm reading between the lines way too much or if i'm being dumb, clueless, and missing out on opportunities.