1. you are going to school later, right? 2. you are going to school later, aren't you? 3. you are going to school later, are you? ===> In my opinion 1) and 2) are correct and replaceable. 3) is wrong because you have to use a contrary question tag.

Thank you tumpliner. But what if I don't expect an affirmative answer but a pure enquiry? In other words, how can I change the sentences above so that they can be equivalent to the setence that are you going to school?

You can actually use #3, but it has a different meaning from the other two. You are asking the same question in all three, but in the first two you are expecting or desiring an affirmative answer. That would be the normal view of a parent asking their child, for example. The third one is expressing surprise or mild disapproval that the person is going to school. It is possible that the child is sick-- the parent does not want the child to go to school-- but it looks like the child wants to go to school and is getting ready to do so.

thank you~

You can use both of 1,2. Both have the same meaning. Also, I often hear the first sentence as spoken language. You cannot use the third sentence, wrong question tag.