You shouldn't create a new StringBuffer each time a subject tag is encountered. You should create it exactly once, and then clear it each time a student tag is closed.
[0] You have major typos in the xml. And then, you have not cleaned up the code StudentInfo or PageInfo class, you've to pick one and use consistently.
[1] Then apart from those, the logic of the sax parsing is clearly defective. Here is a major rewrite based on it (I pick PageInfo).