It will depend on your build really. I started getting a tummy from about 8 weeks with all of mine, but because I'm overweight anyway it didn't always click with people that it was baby rather than belly until I was quite far down the road. I noticed that a slim built Mum at my school was pregnant when she was 9 weeks, but missed another equally slim one until she was 4 months, and then only because we went out and she wore something quite tight!
12 year olds can be quite self absorbed though, so it might be a while.
I couldn't do my trousers up at 13wks, this was despite being sick as a dog and eating barely anything for the previous 7wks.
Despite this, the 10year olds at school didn't notice till their mums told them when I was about 41/2mths.
Good luck with the pg!
I am 11 weeks and am already showing, I seem to have popped this last week but if I wear baggy tshirts nobody can tell, you will just have to be cunning with what you wear if you start showing before you want to tell. My friends found out before I started showing simply by me not drinking/smoking and refusing certain foods so be extra careful, sometimes you don't realise you are being obviously pregnant!