HTTP Status 500 - [PersistenceUnit: default] Unable to build Hibernate SessionFactory

clase Carreras 

package org.openxava.myschool1.model;

import java.util.*;

import javax.persistence.*;
import org.openxava.annotations.*;

@Entity
public class Carreras {

    @Id @Hidden
    @GeneratedValue(strategy=GenerationType.IDENTITY)
    private int id;

     public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    @Id @Column(length=5) @Required  
      private String codigo;

     public String getCodigo() {
        return codigo;
    }

    public void setCodigo(String codigo) {
        this.codigo = codigo;
    }

    public String getCarerra() {
        return carerra;
    }

    public void setCarerra(String carerra) {
        this.carerra = carerra;
    }

    @Column(length=45) @Required  
      private String carerra;


    @OneToMany(mappedBy="carreras")
    private Collection<Pupil> pupil;

    public Collection<Pupil> getPupil() {
        return pupil;
    }

    public void setPupil(Collection<Pupil> pupil) {
        this.pupil = pupil;
    }

}



clase  Pupil

package org.openxava.myschool1.model;

import java.util.*;

import javax.persistence.*;

import org.openxava.annotations.*;


@Entity
public class Pupil {

    @Id @Hidden
    @GeneratedValue(strategy=GenerationType.IDENTITY)
    private int id;

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    @Id @Column(length=5) @Required   
    private String number;

    public String getNumber() {
        return number;
    }

    public void setNumber(String number) {
        this.number = number;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    @Column(length=40) @Required
    private String name;



    @DescriptionsList
       @ManyToOne
        private Carreras carreras;

    public Carreras getCarreras() {
        return carreras;
    }

    public void setCarreras(Carreras carreras) {
        this.carreras = carreras;
    }



}