telling boss I'm preg

Emily
So I'm a teacher and I am 8 weeks pregnant. Today was our last day of school for summer break--other than one day of post planning next week. When we come back to school in August, I will be 18 weeks. I'm not sure whether to tell her next week, or wait until I come back. I would hate to tell her half way through my pregnancy because of our relationship and long term sub planning, but I also don't wAnt to jinx it. 
I've been to the doctor 4 times already. (I have 2 previous losses) last time I went I was 6 weeks 2 days. Everything looked perfect and we saw a strong heart beat of 122 bpm. 
What do y'all think?