you can use Linq Xml
XDocument doc = XDocument.Load(xmlFilePath);
XElement school = doc.Element("School");
school.Add(new XElement("Student",
new XElement("FirstName", "David"),
new XElement("LastName", "Smith")));
doc.Save(xmlFilePath);
Edit
if you want to add Element to Existing <Student>
, just add an Attribute before
school.add(new XElement("Student",
new XAttribute("ID", "ID_Value"),
new XElement("FirstName", "David"),
new XElement("LastName", "Smith")));
Then you can add further Details to the Existing <Student>
by search -> get -> add
XElement particularStudent = doc.Element("School").Elements("Student")
.Where(student => student.Attribute("ID").Value == "SearchID")
.FirstOrDefault();
if(particularStudent != null)
particularStudent.Add(new XElement("<NewElementName>","<Value>");