You can't just post up your code and an error message every time you get a error message - fixing them is a part - and an important part - of producing code.
So look at the message:
Call to undefined function mysql_error()
And at the location:
C:\xampp\htdocs\cecri karikudi\addstudents.php:48
which gives you the file name and the line within the file at which the problem occured.
The message is pretty explicit: the function doesn't exist, and a very quick google:
Call to undefined function mysql_error() - Google Search[
^] tells you why, and what to do about it.
At a guess based on your questions and code so far, you are finding code on the internet, slamming it together and hoping that it will all just work together: that isn't going to happen. You would almost certainly find it quicker and easier to write the code yourself - this isn't complicated stuff and it was almost certainly all covered in your last tutorial period...