Google-sheets – Write cell value based on the URL of another sheet

You need to use the indirect() formula. So in cell B1 you need to put this formula:

=indirect(A1&"!B4")

Since the content of L7 is going to be overwritten, spreadsheet formulas cannot perform such reset. You need a script that has the spreadsheet keys of all the spreadsheets to be changed, and has the authority to edit them (i.e., the Google account under which it runs has this authority). It can be either a stand-alone script, or bound to your master spreadsheet, whichever is more convenient. Also, you'll need to know the name of the sheet within the spreadsheet in which the cell L7 is located; apparently it's "FailingStudents".

function reset() {
  var keys = ['....', '....' ]; // list of spreadsheet ids
  for (var i = 0; i < keys.length; i++) {
    var ss = SpreadsheetApp.openById(keys[i]);
    var sheet = ss.getSheetByName('FailingStudents');
    sheet.getRange('L7').setValue('No');
  }
}

Once you run this, the cell FailingStudents!L7 will be set to No in all listed spreadsheets.