Storing heterogeanous class type in a variable

I'm a beginner with c++ :), I simplified my problem to this:

I have a class Person

class Person{
    string name;}

And another class Student that inherits Person

class Student : public Person{
    string schoolName;}

And then I have a class Library that must have a different comportement depending on the input

class Library{
   void checkBeforeEntry(Person* p){
       cout << "You must be a student to enter" << endl;}
   void checkBeforeEntry(Student* s){
       cout << "You can enter" << endl;}}

At some point in my programm I'm storing a person like that: Person* guy; because at that point I don't know yet if he is a simple person or a student.

But due to that I'm "loosing" the information about the real type of the object and I have tried this:

    Person *p = new Person();
    Student *s = new Student();
    Person *g = new Student();

    lib.checkBeforeEntry(p);
    lib.checkBeforeEntry(s);
    lib.checkBeforeEntry(g);

The Output was:

You must be a student to enter
You can enter
You must be a student to enter

But I want the last statement to be "you can enter". I used a bit of polymorphism before to adapt the comportement of the derived class but here I'm trying to change the comportement of a third party class (library) so the situation is not the same as I am used to. Do you know how to solve this or if I should use another design ?