I am saying that is you really want to understand this, you have to think about it yourself. Read the code. Don't expet to understand it just from someone else explaining it.
Also, I expect your ultimate goal is not to "to get the response submited by student in javascript". I hope your ultimate goal has something to do with education.
I learnt about the code steps by putting debug statements in the code and clicking buttons whilst monitoring the contents of tables. Once I (think) I had my head around it I concluded that it is complex and it is complex because It does a lot of clever and necessary stuff. Vishal, I reccomend you read through the Unit testing code as well, it shows some information on the steps that occur when a test is taken.