I'm 17, ftm, and live in the US. I got my teachers to call me by my chosen name simply by sitting down and having a talk with them on the first day of school. Surprisingly, each of them were accepting and accept me for who I am, and do call me Liam, as I have asked them to. I've done it at each school I've attended, and so far it's been succesful. I would try asking that teacher if he can address you by that name because it's your nickname, and explain to him that it would make you feel more comfortable if he did so. Try going to administrators or counselors at your school, and try talking to them as well. it worked for me.