Forgive me as I am still relatively new to the CA community.
First, I am unsure if having multiple "Students" elements is normal or if it was just repeatedly pasted, but I assumed the latter, and reduced it down to just one set. For that, a line of PowerShell should get you what you are looking for:
[xml](Get-Content -Raw '.\Stack(Sample xml data in csv).csv') | Select-Object -ExpandProperty "Students" | Select-Object -ExpandProperty "Student" | ConvertTo-Csv -NoTypeInformation
This pulls in the information like this:
"StudentID","Name","Marks"
"1","John Smith","200"
"2","Mark Johnson","300"
"3","Nitin Tyagi","400"
And should hopefully get you going in the right direction.