Starting from a Google Spreadsheet containing links to student response documents, this creates a single document containing all of their responses.

concatenateStudentResponses.gs

function addMenu() {
  var ui = SpreadsheetApp.getUi();
  ui.createMenu('Student Responses').addItem('Concatenate all responses', 'concatenateResponses').addToUi();
}

function concatenateResponses() {
  var responsesDoc = DocumentApp.create('Student Responses');
  var responsesDocID = responsesDoc.getId();
  var sheet = SpreadsheetApp.getActiveSheet();
  var columnContainingLinksToDocuments = 6;
  var lastRow = sheet.getLastRow();
  var docIDs = [];
  // loop through the cells to get the document IDs for all the response documents
  for (var x=2; x<=lastRow; x++) {
    var docLink = sheet.getRange(x, columnContainingLinksToDocuments).getValue();
    var splitDocLink = docLink.split('/');
    var docID = splitDocLink[7];
    docIDs[x-2] = docID;
  }
  mergeGoogleDocs(docIDs, responsesDocID);
}

// Retrieved and modified from from https://ctrlq.org/code/19892-merge-multiple-google-documents
function mergeGoogleDocs(docIDs, responsesDocID) {
  var baseDoc = DocumentApp.openById(responsesDocID);
  var body = baseDoc.getActiveSection();
  for (var i=0; i<docIDs.length; i++) {
    var studentNumber = i+1
    body.appendParagraph('Student Number ' + studentNumber).setBold(true).setItalic(true);
    var otherBody = DocumentApp.openById(docIDs[i]).getActiveSection();
    var totalElements = otherBody.getNumChildren();
    for( var j = 0; j < totalElements; ++j ) {
      var element = otherBody.getChild(j).copy();
      var type = element.getType();
      if( type == DocumentApp.ElementType.PARAGRAPH )
        body.appendParagraph(element);
      else if( type == DocumentApp.ElementType.TABLE )
        body.appendTable(element);
      else if( type == DocumentApp.ElementType.LIST_ITEM )
        body.appendListItem(element);
      else
        throw new Error("Unknown element type: "+type);
    }
    body.appendHorizontalRule();
    body.appendPageBreak();
  }
}