error: passing as 'this' discards qualifiers

[miaOnDeLine]

I been having this "error: passing as 'this' discards qualifiers" which i didnt understand it clearly. Any idea where i mistaken? I have surfing the net to look for some clues but unlucky though...Here are some code of mine. I have other classes too...

class Subject 
{
	private:
	string subCode;
	vector<string>facOffer;
	set<Student>stuTaken;
	public:
	Subject();
	Subject(string subCode,set<string>facOffer1);
	string getSubCode();
	void setStuTaken(Student stud);
	void dropStuTaken(Student stud);
	void print();
	
};


void Subject::print()
{
	cout<<"Subject code   :"<<subCode<<endl;
	cout<<"Faculty Offer : \n";
	for(int j=0;j<facOffer.size();j++)
	{
		cout<<j+1<<" . "<<facOffer[j]<<endl;
	}
	cout<<endl;
	cout<<"Registered student : \n";
	if(stuTaken.size()==0)
	{ cout<<" ---ZERO student registration.---\n";}
	else
	{	for(int j=0;j<stuTaken.size();j++)
	     {	
		set<Student>::iterator it=stuTaken.begin();	
		while(it!=stuTaken.end())
		{
		cout<<"Student Record no. "<<j+1<<" . \n";
		it[j].print(); ///////error line
                it++;
		}
		
	       }	
	}
}


void Subject::dropStuTaken(Student stud)
{
	set<Student>::iterator it=stuTaken.begin();	
	while(it!=stuTaken.end())
	{	if(it->getName()==stud.getName()) //////error line
		{
			stuTaken.erase(it);
			cout<<"Deleted";
		}
		it++;
	}
}

Thanks in advance.

[Bazzy]

the problem is in class Student, check your const functions

[miaOnDeLine]

My Student class

class Student
{
	private:
	string name;
	string faculty;
	vector<string>subject;
	
	public:
	Student();
	Student(string name,string faculty);
	void registerSubject(set<string>sub);
	void print();
	string getName();
	string getSubject(int i);
	void dropStudFromSubject(set<string>sub);
	int getSubjectSize();
};