Google script to get averaged scores and emails fr

I need a Google script (GAS) to retrieve both emails and total scores from a Google Form that was turned into a quiz.

Within GAS you can get the score for each question, but I need to get the score of all questions and then average out the final score (each quiz has 20 questions). I also need the email of each person who finished the quiz. These quizzes are done within institutions so the "Collect email addresses" is selected.

I need this and NOT the "responses spreadsheet" because I will be making hundreds of quizzes and I need to put all the grades in one spreadsheet, as a summary. I do not want to have hundreds of response spreadsheets.

What I have up to now is below, but I cannot seem to get the average score of each quiz and put it beside the email in a spreadsheet. Any help would be appreciated.

function getPoints() {
var form = FormApp.openById('ID');
 var formResponses = form.getResponses();
 var formItems = form.getItems();

 for (var i = 0; i < formResponses.length; i++) {
  var formResponse = formResponses[i];
   var email = formResponse.getRespondentEmail();

/* I need to get all emails from those who responded, 
   not just one and put them in column A.*/

   var s = SpreadsheetApp.openById("ID").getSheetByName("Sheet1");
   var sr = s.getRange("A:A").setValues(email);
 }

 for (var j = 0; j < formItems.length; j++) { 
   var item = formItems[i];
    if (item.getType() === item.getType().TEXT){
    var points = item.asTextItem().getPoints(); 
    var itemResponse = formResponse.getGradableResponseForItem(item);
    var answer = itemResponse.getResponse();
    var sc = itemResponse.getScore();

    /* I need to get all the scores, not just one, and then average
       them, and them put them in column B, beside the corresponding
       email in column A. */

    var s = SpreadsheetApp.openById("ID").getSheetByName("Sheet1");
    var sr = s.getRange("B:B").setValues(sc);
    }
 }
}

Your script is very close.

As there isn't a method to get at once all the item scores, one alternative is to build an array of item scores, then calculate the average for the each quiz (submitted response).

Example:

The following script, intended to be used as a bounded script, include three functions:

onOpen Adds a custom menu
showAvarage the main function that sets the form to be processed and calls the function that creates a 2D array. It could be used to send the values to a spreadsheet, but for simplicity this was omitted.
calculateAvarage Collects the respondents emails and calculates the average score for each quiz (response submission)

function onOpen(e) {
  var ui = FormApp.getUi();
  var menu = ui.createMenu('My Menu')
  .addItem('Average', 'showAverage')
  .addToUi();
}

function showAverage(){
  // Works for bounded scripts called from custom menus or the script editor
  var form = FormApp.getActiveForm(); 

  Logger.log(calculateAverage(form));
}

/**
 * Calculate the average score for each submitted response
 *
 * @param {Object} form      The form to be processed.
 * @return {Array}           2D array. First column respondent email, 
 *                             second column response average.
 */
function calculateAverage(form){

  var formResponses = form.getResponses();

  // If there aren't submitted responses, exit.
  if(formResponses.length == 0) return 'No responses';

  // Initialize output
  var output = [];

  for(var i = 0; i < formResponses.length ; i++){
    var itemResponses = formResponses[i].getGradableItemResponses();

    // Initialize scores array. Later it will be used to calculate the score average.
    var scores = [];
    for(var j = 0; j < itemResponses.length; j++){
      var score = itemResponses[j].getScore();
      scores.push(score);
    }

    // Average calculation. Reference https://stackoverflow.com/a/10624256/1595451
    var sum = scores.reduce(function(a, b) { return a + b; });
    var avg = sum / scores.length;

    // Add row
    var email = formResponses[i].getRespondentEmail();
    output.push([email,avg]);
  }

  return output;
}

Reference