Telling work that you're pregnant

Laura
I'm 10 weeks today and work as a primary school teacher in the UK. 
I need to tell my head that I'm pregnant but there's so many changes happening in the school that I can't seem to find the right time! 
I'm also an NQT and feel even more nervous about telling the head that I will be back in September for one term only before finishing for maternity. 
How is best to approach the situation? Need to tell them soon as I have a scan on the 4th!