In desperate need of help

Hi all,

I just found out over the weekend that I am 5 weeks pregnant. I am currently a casual high school teacher and have been offered part time teaching in 2020. The principle has drafted a timetable for me already, and desperately wants me to do full time but I can’t due to university studies. I don’t know when I should inform the school that I will be due in July 2020. When is the right time? I will admit I am intimidated by the principle and don’t want to leave it too late, however I’m not obligated to tell my work yet, right?