Implementing a CRUD using an Interface

What is the best approach to implement a CRUD on the BL using interface that will be used to abstract DAL operations? I need your opinion guys..

Here's my draft..

Data Entities that are mapped in the database table

 public class Student
 {
    public string StudentId { get; set; }
    public string StudentName { get; set; }
    public Course StudentCourse { get; set; }
 }

 public class Course
 {
    public string CourseCode { get; set; }
    public string CourseDesc { get; set; }
 }

I created an CRUD Interface to abstract the object's operations

public interface IMaintanable<T>
{
   void Create(T obj);
   T Retrieve(string key);
   void Update(string key);
   void Delete(string key);
 }

And then a component that manages the Entity and its operations by implementing the interface

public class StudentManager : IMaintainable<Student>
{
    public void Create(Student obj)
    {
        // inserts record in the DB via DAL
    }

    public Student Retrieve(string userId)
    {
        // retrieveds record from the DB via DAL
    }

    public void Update()
    {
        // should update the record in the DB
    }

    public void Delete(string userId)
    {
        // deletes record from the DB
    }
}

sample usage

    public void Button_SaveStudent(Event args, object sender)
    {
        Student student = new Student()
        {
           StudentId = "1", StudentName = "Cnillincy"
        }

        new StudentManager().Create(student);   
     }

as you can see, there is quite an abnormalities on the update method

    public void Update()
    {
        // should update the record in the DB
    }

what should this method have to update the objects property? should I inherit the Student?

    public class StudentManager : Student , IMaintainable<Student>
    {
        public void Update()
        {
            //update record via DAL
         }
    }


    public void Button_SaveStudent(Event args, object sender)
    {
        Student student = new StudentManager();
        student.StudentId = "1";
        student.StudentName = "Cnillincy"
        student.Update()
    }

Or should I just contain the Student class as an attribute of the Student manager?

     public class StudentManager : IMaintainable<Student>
    {
        public Student student { get; private set };

        public void Create() {}
        public void Update() {}
        public void Retrieve() {}
        public void Delete() {}
    }

Which more appropriate? What about the interface? Any other suggestions guys? thanks..C