这儿是他的班。(right, but we usually omit the words "儿",and directly say :这是他的班," )
他是这儿的班.(right, but here 他 no more means a man, it means a class, so this sentence's meaning become : It's the class belong here. And "他" should be changed into "它"
他们都也汉语学生.( you can only use one word 都 or 也, and add the " 是" in the sentence. so there would be two sentences of different meaning. One is :他们都是汉语学生, here 他们 means the two person you mentioned before. Another is : 他们也是汉语学生,maybe there are more than 2 person and the rest are Han language learner too.)