Python: "str" object is not callable

This code is for a project I'm working on; I'm apparently trying to call a string. The program is designed to print quiz scores from an SQLite database; method 1, which is what I'm having problems with, should give the most recent records of all students of the right class and difficulty level. The database looks like this;

CREATE TABLE Scores ( [Key] INTEGER, Score INTEGER NOT NULL, PupilName TEXT NOT NULL, Difficulty BOOLEAN NOT NULL, ClassNumber VARCHAR, DateTime DATETIME );

and the code (with the odd bit of debug) like this:

import sqlite3

#import collections
print ("Welcome to the Arithmetic Challenge high score table.")
def MainLoop():
    DisplayType = input ("Please enter: 1 for highest scores by student in alphabetical order; 2 for highest scores, highest to lowest; 3 for average scores, highest to lowest. ")
    Difficulty = input ("Enter difficulty level.")
    ClassNumber = input ("Enter class number.") #Input of variables
    conn = sqlite3.connect("QuizStorage")
    c = conn.cursor()#Connects
    c.execute("SELECT * FROM Scores")                            
    PupilsEligible = c.fetchall()#Finds pupils
    #count = collections.defaultdict(int) #?
    #print (count)
    print (PupilsEligible)
    DifficultyInt = int(Difficulty)
    if DisplayType == "1":
        print (DifficultyInt)
        c.execute("SELECT * FROM Scores WHERE Difficulty = (?) AND ClassNumber = (?) ORDER BY # DESC LIMIT 1 ORDER BY PupilName"(DifficultyInt, ClassNumber))
        data = c.fetchall()
        print (data)
    elif DisplayType == "2":
        c.execute("SELECT * , MAX(Score) FROM Scores WHERE Difficulty = (?) AND ClassNumber = (?) GROUP BY PupilName"(DifficultyInt, ClassNumber))
        data = c.fetchall()
        print (data)
    elif DisplayType == "3":
        c.execute("SELECT * , AVG(Score) FROM Scores WHERE Difficulty = (?) AND ClassNumber = (?) GROUP BY PupilName"(DifficultyInt, ClassNumber))
        data = c.fetchall()
        print (data)

    else:
        print ("That's not a valid answer.")
    for row in c:
        print (row)
    conn.close()
    MainLoop()
MainLoop()

Please note that this code has various relics of earlier attempts that haven't yet been pruned off. Sorry if this makes it unclear. When I try to run it, I get this :

Welcome to the Arithmetic Challenge high score table.
Please enter: 1 for highest scores by student in alphabetical order; 2 for highest scores, highest to lowest; 3 for average scores, highest to lowest. 1
Enter difficulty level.2
Enter class number.3
[('Alice Mann',), ('Fred Pratt',), ('John Smith',), ('Karen James',)]
2
Traceback (most recent call last):
  File "H:\My Documents\CA 2\ACHighScore 0.5.5.py", line 37, in <module>
    MainLoop()
  File "H:\My Documents\CA 2\ACHighScore 0.5.5.py", line 19, in MainLoop
    c.execute("SELECT * FROM Scores WHERE Difficulty = (?) AND ClassNumber = (?) ORDER BY # DESC LIMIT 1 ORDER BY PupilName"(DifficultyInt, ClassNumber))
TypeError: 'str' object is not callable

I've looked at the other similar errors and been left none the wiser. What exactly does this error message mean, and how can it be avoided?