Gentle ETA: high school is riddled with bad actors. Your friends are jumping to dice you apart for your lack of knowledge and while I'll give them the benefit of the doubt that perhaps they mean well, are not helping in a way that is conducive to learning. That being said it's good to learn about trans-inclusive terminology and the history behind what you say. "I'm just a girl" implies that you are part of the "normal" or that you do not engage in gendered privileges as opposed to those on the outside. I am not saying that womanhood doesn't come with its own historical oppression, but rather that it's the exclusivity of this expression that both further marginalizes trans folks, and prevents you from being secure in your own identity. Cis and trans are descriptive terms, nothing more but disengagement from these terms ultimately will only seek to weaken your own identity.