Teacher treat others better

Sometimes our side-teacher treat other kids better. An example is when we were having music. Everyone got to groups. 2 were in every group. Everyone got to play piano. Our teacher walked to a group and said: Its so sad that you are here with these kids. You have talent! And another example is when we was late to lunch. First i came late and the teacher was angry. And one day i came late to lunch again but with the group he talked to. He was angry. After we ate lunch, one of the kids in the group said: Wow he has never been so angry to me. You know when i come late he isnt angry at all. Another example is when the group was playing mikado but with big, big sticks. My friend likes to ruin things so one time he took one of the sticks. When he gave the stick to them one of the persons from the group took a fucking stick and slammed him with it so hard! He was in great pain and he saw the teacher. He called for him and said what they had done. Even i supported him. But he seemed so calm! He was just like "ok but there has to be a reason." I was boiling. A REASON? Violence is never accepted. Because we were young and he was a teacher the group was not even said anything like "dont slap kids with sticks" He just walked away! Is this normal, and if it is wrong, tell me what i can do. AND BTW, the group are very nice to the teacher but to other kids! They are so mean! They bullied someone with some kind of mental disorder by not letting him join the game and when they let him join, the group CRUSHED him with a mattress.. And they swear. Alot.