It looks like that you added calificacion column later in the database.
I would do one of the following:

1. Uninstalling and re-installing your app.
2. The best and better approach is to drop and recreate Estudiantes table in onUpdate method, and increase the db version wheneveryou change the schema.

This is the class that I´m using

public class DBHelper extends SQLiteOpenHelper {

    private static final String DB_FILE = "Database.db";
    private static final String TABLE = "Estudiantes";
    private static final String FIELD_ID = "id";
    private static final String FIELD_NAME = "nombre";
    private static final String FIELD_GRADE = "calificacion";

    public DBHelper(Context context){
        super(context, DB_FILE, null, 1);
    }

    @Override
    public void onCreate(SQLiteDatabase db) {
        String query = "CREATE TABLE " + TABLE + "(" +
                FIELD_ID + " INTEGER PRIMARY KEY, " +
                FIELD_NAME + " TEXT, " +
                FIELD_GRADE + " INTEGER);";
        db.execSQL(query);
    }

    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
        String query = "DROP TABLE IF EXISTS ?";
        String[] params = {TABLE};
        db.execSQL(query, params);
    }

    public void save(String nombre, int calificacion){
        SQLiteDatabase db = getWritableDatabase();
        ContentValues valores = new ContentValues();

        valores.put(FIELD_NAME, nombre);
        valores.put(FIELD_GRADE, calificacion);

        db.insert(TABLE, null, valores);
    }

    public int delete(String nombre){
        SQLiteDatabase db = getWritableDatabase();
        String clause = FIELD_NAME + " = ?";
        String[] args = {nombre};
        return db.delete(TABLE, clause, args);
    }

    public int find(String nombre){
        SQLiteDatabase db = getReadableDatabase();
        String filtrito = FIELD_NAME + " = ?";
        String[] args = {nombre};

        Cursor c = db.query(TABLE, null, filtrito, args, null, null, null);
        int result = -1;

        if(c.moveToFirst()) {
            result = c.getInt(2);
        }

        return result;
    }
}

This is the error message I get:

E/SQLiteLog: (1) table Estudiantes has no column named calificacion
E/SQLiteDatabase: Error inserting calificacion=80 nombre=Fer
android.database.sqlite.SQLiteException: 
table Estudiantes has no column named calificacion (code 1): , while compiling: 
INSERT INTO Estudiantes(calificacion,nombre) VALUES (?,?)