Honestly I was kind of amazed by how quickly people at my school got the memo, even if they hadn't been personally informed. I sent out a mass letter on Facebook to let everyone I possibly could know right before Christmas break, I believe, so there was a long period where I wasn't sure what my experience for the rest of the year would be like. Once teachers and friends started calling me Andrew, the confused ones either just shrugged and went along with it or went "'Andrew?' What? Did I miss something?" to which either myself or another, more informed classmate would fill in the blanks as needed. The nice thing is, the less people know you, the easier it is for them to switch over pronouns and names; at least, that's what I've found in my experience.
Good luck, I know it takes a lot of courage, coming out in high school; push through and I'm sure you'll get where you need to be!
EDIT: Also, if you are confident that school administrators, such as a class counselor, will be on your side if you let them know, I highly encourage it. I CC'd my senior counselor and vice principal in the email I sent out to my teachers, and they were sure to let me know that they were here for me. When I asked if I was free to use the gender-confirming bathrooms of my choosing, they were adamant that that was a basic civil right that I was free to take advantage of.