Java inheritance of Hibernate Entities

Background: I am building a web application that allows Teachers to Quiz their students with many analytical features to help gauge individual/class performance on test/quizzes. Currently, I am in the middle of redesigning the MySQL DB Schema and you can see a rudimentary EEF diagram of what I've built thus far included:

I am actually using Hibernate to create the connection from my backend to my DB but this EEF Diagram does a good job of showing the relationships between the tables.

The Problem I currently have separate tables for Admins, Students, and Teachers because all 3 will have unique fields even though, currently, they have pretty much similar fields. The issue with having 3 different tables, is that I want Teachers and Students to share some functionality. For instance, a Teacher should be able to take a quiz just like a student(for testing purposes). My first instinct is to make a User class that both Student and Teacher extend:

 public class User{
...
}
@Entity
@Table(name="student")
public class Student extends User{
...
}
@Entity
@Table(name="teacher")
public class Teacher extends User{
...
}

This approach accomplishes what I want to do with my quiz taking service:

public QuizInstance takeQuiz(User u, QuizStructure struct){
...
}

but I am unsure how I create the mapping relationships when there is polymorphism involved. A good example of this issue is with the registerUser service:

public void registerUser(User u, String password) {
        if(u instanceof Teacher) {
            //register User into Teacher Table..
        }else if(u instanceof Student) {
            //register User into Student Table...
        }
        new LoginService().createLogin(u, password);
    }
public class LoginService{
    ...
    public void createLogin(User u, String password){
        Login l = new Login();
        l.setEncrPass(encrypt(password));
        l.setEmail(u.getEmail());
        if(u instanceof Teacher)
            l.setRole("teacher");
        else
            l.setRole("student");
        l.setUser(u);

        //Save Login into Login Table in DB
    }
}
@Entity
@Table(name="login")
public class Login {

    @Id
    @GeneratedValue(strategy=GenerationType.IDENTITY)
    @Column(name="user_id", unique=true, nullable=false)
    private int id;

    @Column(name="email", nullable=false)
    private String email;

    @Column(name="encrypted_pass", nullable=false)
    private String encryptedPass;

    @Column(name="role", nullable=false)
    private String role;

    //How do I map User if it can link to either the Teacher table or the Student Table???
    private User user;
}

I question if this is the best approach to solving the issue. I can't help but shake that the design of my DB Schema is an issue, security and structure-wise. Can someone tell me if the DB Architecture I've constructed is poor craftmanship and if so share some examples/resources that better help illustrate more secure Architectures for DBs.