Whew, Rachel found her voice in this film, didn't she? I'm pretty sure everyone saw this coming, especially anyone who's dated someone who had a super close friendship. Her frustration is understandable, and despite how she blew up towards Elle, her feelings are valid. The problem stems from her demanding that Lee tell Elle to back off. No one has a right to tell their partner to stop hanging out with someone they care about, especially if that person has been part of their lives for way longer than you've known they exist. Instead, she could've shared her irritation with Lee and let him decide how he wants to act in accordance to her feelings, but that may be hoping for too much with high school students.
Then there's Lee's decision to not tell Elle about Rachel's feelings, which is both him choosing "a side" and a selfish move. He wants to make Rachel feel like he would push Elle away for her without actually doing so, because he doesn't want to stop hanging out with Elle. This leads to Rachel believing that Elle is purposely interrupting her time with Lee and she lashes out in resentment. When the truth comes out, Rachel rightfully breaks up with Lee for deceiving her and essentially choosing Elle over her.
Rachel and Lee get back together in the end, but I can't help but feel like it was the wrong choice for everyone all around. If Elle goes to Berkeley with Lee, the three could fall into the same toxic dynamic as before. Despite promising to put her first, Lee is obviously terrible at pulling himself away from Elle unless they're isolated from one another, which doesn't make either of them happy. And if Elle doesn't go to Berkeley with Lee, I have a feeling that their attempts to make long-distance work with their friendship would take up enough time to still put a strain on their respective relationships. Rachel doesn't seem like she'd let Lee mess with her for a second time, so "Leechel" fans will have to cross their fingers that these two crazy kids can make it work during undergrad.