I don't trust that teachers actually check their email during the summer, so what I do (did this yesterday too as it was the first day) is get to all of my classes earlier than everyone else, which isn't too hard because most people socialize since it's the first day of school. I basically tell them I'm going through a transition, my name isn't legally changed yet, but I present as Jacob/male blah blah. If they don't agree and go all "I need a note from the office first" I ask them to call me by my last name for the time being.
However, this year was a tad different. My name will really be changed sometime next month, so I just told my teachers "My name in the school system is Brianna, but it's supposed to be Jacob. It won't be changed until late October." The reaction from all of them was "How could they make such a mistake?!" so I told them I was changing it, the school didn't make a mistake. . .It was quite funny actually

I hope it all works out for you though! Best of wishes