I would suggest that there are a couple of levels to the conflict between Elena and Gail. On one hand, Elena is in a lower social "grouping" than Gail. Gail is at a higher level of the social stratification of P.S. 13. Gail is the one who is able to serve as "the Queen Bee," giving Elena her nickname of "Skinny Bones" and having a gaggle of followers to ensure that her taunts of Elena resonate:
"Hey, Skinny Bones, pump it, girl. Ain't you got no energy today?" Gail, the biggest of the black girls who had the other end of the rope, yelled, "Didn't you eat your rice and beans and pork chops for breakfast today?"
The other girls picked up the "pork chop" and made it into a refrain: "Pork chop, pork chop, did you eat your pork chop?" They entered the double ropes in pairs and exited without tripping or missing a beat. I felt a burning on my cheeks, and then my glasses fogged up so that I could not manage to coordinate the jump rope with Gail. The chill was doing to me what it always did, entering my bones, making me cry, humiliating me.
On one level, the conflict between both Gail and Elena is a social one. Gail has power and Elena does not.
Part of the reason that this power imbalance exists is in the form of community and solidarity. Gail has the support of other African- American girls. Elena is, for the most part, alone. Elena is a newly transported immigrant in the social world of the high school. Gail is more rooted and thus has the advantage of community. This helps to reveal another layer to the conflict between both. Gail does not have to pretend to be anything. Elena is shown to be alone and isolated, and in her desire to assimilate or in the very least not stick out, Elena must endure Gail's harassment. She accepts the name "Skinny Bones" out of refusing to correct anyone. Elena has no social or ethnic- based reinforcement to challenge Gail. The same discrimination and prejudice that Gail would experience in another part of New York is what Elena experiences at Gail's hands and in the present social setting.
Finally, Gail experiences conflict with Elena out of her own personal insecurities. Experiencing some of the most challenging elements of puberty and adolescence, Elena derides her own "flat chested" state and envies the more full forms of the African- American girls, like Gail. This also feeds into the larger element that the girls like Gail feel more at home in New Jersey, more accustomed the weather and more at ease with the surroundings. This is something that Elena lacks: "I hated the city, especially in winter. I hated Public School Number 13. I hated my skinny flat-chested body, and I envied the black girls who could jump rope so fast that their legs became a blur. They always seemed to be warm while I froze." Already "humiliated" by her different experience, this enhances another layer of conflict between both girls.