On any legal paperwork (for school, medical, anything really) you have to put your legal name and legal sex/gender. Not doing so I think may be fraud (or some other crime?), even though you identify as otherwise. I've learned this through having to put down my legal name and sex on medical paperwork. Its nothing personal, just the law. Once you get everything legally changed over in paperwork (name change, gender marker changed, reflected on social security card, etc) then you can begin writing what your true name and sex are on any new paperwork that comes your way.
However, in a social setting absolutely you can use the name you feel is right for you and your gender appropriate pronouns. For example, in any doctor's office I go to they know my gender pronouns and what the name I like to use is, and they call me that appropriately orally. Some even correct new people about it. However, on legal paperwork at their offices I must put my name and gender marker as is by law, and they do sometimes apologize about having to do that if I'm signing things or filling out things in front of them. So its not a total punch in the face and doesn't have to be ^.^ Example, a typical interaction goes as follows:
Medical worker: "Hey, Sumi! How you doin', man?"
Me: "I'm great! I have to fill out some new paperwork today!"
Medical worker: "You know we have to put your legal name on that, right? I'm so sorry
sad "
Me: "Haha, I know, I know. That's the legal reality!"
Medical worker: "Haha, yeah
sweatdrop "
Hope that clears some things up for you!
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