I started wearing a training bra when I was in Year 6, because all my mates were wearing one. Now, I know that I didn't actually need to start wearing one until I was in late Year 7. The trick with something like this is to act confident, whether you are wearing a training bra or not. Stand up straight and keep your chin up! If you feel awkward wearing a training bra, then that probably means you aren't ready. In your PE uniform (if your school has one!), bra lines are quite obvious, but it is for everybody.
You may not be ready to wear a training bra yet, but you may need to wear one. An option for this sticky situation is to buy one of those BONDS singlets which have a training bra built into them. These are so comfy and not obvious at all! I wear mine as a pajama top, and sometimes for sports or just casual wear.
If you choose to start wearing a training bra, wear it everyday. This way you'll get used to it. It can be soooo uncomfortable at first, (believe me, I would know!) but give it one month and you'll be fine.
A training bra isn't necessary. I know some girls who just didn't wear any training bra, and then all of a sudden BAM! They were wearing proper bras.
I hope I helped you make a desicion!
Ps. Those people who think you are wrong are probably just jealous of you amazingness :)
22 minutes ago