Unfortunately I don't really have a solid answer for you. From my own experience (I was bullied throughout most of elementary school) I would say, it's probably best if you ignore any mean things they say to you.
If they also bully you in a physical way (hitting, pushing, etc) it would probably be best if you hit back harder. I know that this is not what most adults would advice, but this is what my parents always thought me. I also know that this might be easier said then done, especially since I'm a tall guy. For a boy/girl that is much smaller then his/her bullies this might not seem like a smart idea, but in most cases this will stop the physical bullying.
To answer your question "Where to ask for justice?". I'm afraid most adults won't help you, they rarely do when it comes to bullying, so if you really want justice, you're probably going to have to get it yourself. I know that it isn't fair but in most cases, this is the unfortunate truth.
One thing that might also be a solution for you is following a home study. I'm not sure what it's called in English, but what I mean is following classes at home, not through your parents, but through a special education system.
I don't advice running away from your problems (the bullies), but if that's the only solution then a home study might work for you.
I hope this helps you in some way. You may also add me as a friend or follow me on this site, that way you can send me a message if you ever feel like you need to talk to someone. (In case it matters to you; I'm an 18 year old male)