XmlSerializer in inherited objects

[Bluetears76]

Now Hierarchy is that there is a class named "Student"
There are two child classes "GraduateStuden t" and "HighSchoolStud ent".
I worte the following code for serializing the student and
deserializing.
Now the problem is that as i do not know the type of Student while
deserializing so i get an error on line
student= (Student)deseri alizer.Deserial ize(memoryStrea m);

Any help will be highly appriciated
public void SerializeMyStud ent(Student student)
{
NetworkStream netWorkStream=n ull;
try
{
XmlSerializer serializer=new XmlSerializer(s tudent.GetType( ));
netWorkStream=n ew NetworkStream(_ clientSocket);
Stream stream=(Stream) netWorkStream;
serializer.Seri alize(stream,st udent);
}
finally
{
netWorkStream.C lose();
}
}

and the deserializeMess age is like
public bool DeserializeMess age(ref NetworkStream networkStream)
{

XmlSerializer deserializer=nu ll;
int count=0;
Byte []buffer=new Byte[BUFFER_SIZE];
count=networkSt ream.Read(buffe r,0,buffer.Leng th);
if(count <= 0)
{
message=null;
return false;
}

Student student= new Student();
deserializer=ne w XmlSerializer(s tudent.GetType( ));

MemoryStream memoryStream=nu ll;
memoryStream=ne w MemoryStream(bu ffer,0,count);
student= (Student)deseri alizer.Deserial ize(memoryStrea m);
return true;
}