When should I tell my employer about my pregnancy ?

So this is my first pregnancy, and I am also a first year teacher. I start my new job in August, and I have not yet told my employer that I am pregnant. I am worried that they may say the position is no longer available or something. I have heard that schools do not like hiring pregnant women because they have to hire someone else to fill in temporarily. I am 14 weeks now, and will be about 21 weeks when I start my new job August. So I guess I wondering do you ladies think I should tell my employer that I am pregnant, prior to the school year starting ? or Is it wise to wait a few weeks into the school year to mention it? Honestly, any advice on how to tell my employer I am pregnant would be greatly appreciated.