A couple things:
1. If your teachers see cuts and bruises, they have no way of knowing they are self inflicted. Many parents abuse their children, and many children learn early on to lie about how those cuts and bruises happened.
2. Imagine that you walk outside tomorrow and you step on a big rusty nail. It goes really deep into the sole of your foot. You decide that you don't want to be bothered going to the doctor about it and you don't want to take the time to clean it out, so you just put a bandage on it. The next day, your foot really hurts but again, you don't want to address it, so you put a bigger bandage on it. The following day, not only is the foot in even worse condition - swollen and hot and oozing, it really hurts. It hurts so much that you decide to hit yourself on the head with a hammer to distract yourself from the pain in your foot. The day after that, your entire foot and lower leg are swelling. You decide you need a bigger hammer to hit yourself with on the head so you don't think about your foot.
Self-harm is the bandages and the hammers that you are using to cover over and distract yourself from the real problem. The real problem needs real, appropriate help and treatment - cutting yourself to distract from it is never, not ever, going to resolve it or help it.
Please do tell your teachers what is going on. They can help you find someone to help you address the real problem which needs real, appropriate help.