I'm going back to my old high school to see some old teachers (ironically my Psychology teacher). Its probably one of the last places most of us want to be caught, but I'd really love to talk to them and keep in touch. But I went through high school as a female, how would I explain to my mentor that I'm male now?

She is a Psychology teacher, so she may understand to a degree. But I'm sure it will still shock her. And what about her students? I'm sure I'll be interrupting class. Maybe I'm just worried about her recognizing and/or accepting me now that I've made these changes.

teachers always have free periods and lunchtimes so go see her and let her see the new you. 
If statistics are to be believed there may be several other TS in the school population at this very moment so the teacher may welcome a chat about TS things in general.

Email her first. For both coming out and scheduling purposes.

My son just went back to his high school ... he transitioned in college ... he had no problems with teachers and found them to be very warm and supportive