Can't seem to load information from excel file (C# & Excel Interop)

Question

So I have created this program and it should reset the value and type of the cell to 1 and integer on startup. Then when the user wants to add a name, it should read the value and set it as AmountNumD (because it seems to detect it as a double). The program then converts this into an integer AmountNum and uses this in a number of ways leading to the creation of a textbox, progress bar and checkbox on the main form. This works, except when you go to the add name page again, it crashes giving the error: Exception from HRESULT: 0x800401A8.

Below are extracts from my code:

Resetting Cells:

if (File.Exists(StudentDirectory) == true)
        {
            Excel.Application ExcelOApp;
            Excel.Worksheet ExcelOSheet;
            Excel.Workbook ExcelOBook;

            ExcelOApp = new Excel.Application();
            ExcelOBook = ExcelOApp.Workbooks.Open(StudentDirectory);
            ExcelOSheet = (Excel.Worksheet)ExcelOBook.Worksheets.get_Item(1);


            ExcelOSheet.Cells[1, 8] = "Amount Number";
            //deleting old content for Constant Number
            //Excel.Range rngCon = ExcelOSheet.get_Range(2, 8);
            var rngCon = ExcelOSheet.Cells[2, 8];
            rngCon.Cells.NumberFormat = "0";
            rngCon.Cells.Value = "1";

            ExcelOBook.SaveAs(StudentDirectory);
            ExcelOBook.Close();
            ExcelOApp.Quit();
        }
        else
        {
            CreateStudentWorksheet();
            //Goes to a similar section that creates new excel file
        }

Adding students (Note this is on a separate form):

public static string StudentDirectory = "C:\\Users\\Administrator\\Desktop\\Prototype5\\Save Folder\\Students.xlsx";
    //Creating Variables that link to the Student Data Excel File
    public static Excel.Application ExcelApp = new Excel.Application();
    public static Excel.Workbook ExcelBook = ExcelApp.Workbooks.Open(StudentDirectory, 0, false, 5, "", "", true, Excel.XlPlatform.xlWindows, "\t", true, false, 1, 0);
    public static Excel._Worksheet ExcelSheet = (Excel._Worksheet)ExcelBook.Sheets[1];
    //public static ExcelApp.DisplayAlerts = false;

    //Creating a constant number so that the form knows where to put student assets
    int AmountNum;
    double AmountNumD = ExcelSheet.Cells[2, 8].Value;


    public void Constant()
    {
        ExcelApp.DisplayAlerts = false;
        AmountNum = Convert.ToInt32(AmountNumD);
        AmountNum = AmountNum + 1;
        double AmountNumDTwo = Convert.ToDouble(AmountNum);
        ExcelSheet.Cells[2, 8].Value = AmountNumDTwo;
        WritingToExcel(AmountNum);
    }

    private void WritingToExcel(int AmountNum)
    {
        string StudentName = NameBox.Text;
        string StudentPassword = PasswordBox.Text;
        ExcelSheet.Cells[AmountNum, 1].Value = StudentName;
        ExcelSheet.Cells[AmountNum, 5].Value = StudentPassword;
    }

    private void SaveButton_Click(object sender, EventArgs e)
    {
        Constant();
        ExcelBook.Save();
        ExcelApp.Quit();
        this.Close();

    }

What happens in the main form after student details are entered and a button calling this function is pressed:

Excel.Application ExcelApp = new Excel.Application();
        Excel.Workbook ExcelBook = ExcelApp.Workbooks.Open(StudentDirectory, 0, true, 5, "", "", true, Excel.XlPlatform.xlWindows, "\t", true, false, 1, 0);
        Excel._Worksheet ExcelSheet = (Excel._Worksheet)ExcelBook.Sheets[1];
        //Excel.Range NamesRng = ExcelSheet.Cells["A2", "A35"];

        int StartColumn = 138;
        int StartRow = 72;
        int AmountNum;
        double AmountNumD = ExcelSheet.Cells[2, 8].Value;
        AmountNum = Convert.ToInt32(AmountNumD);


        if (AmountNum == 1)
        {
            MessageBox.Show("Please Enter a student first");
        }
        if (AmountNum == 2)
        {
            TextBox TB2 = new TextBox();
            TB2.Name = ("Name" + (AmountNum));
            TB2.Location = new Point(StartColumn, StartRow);
            TB2.Size = new Size(78, 22);
            TB2.ReadOnly = true;
            TB2.Text = ExcelSheet.Cells[AmountNum, 1].Text;
            this.Controls.Add(TB2);
            TB2.BringToFront();
            TB2.Show();
            ProgressBar PB2 = new ProgressBar();
            PB2.Name = ("ProgressBar" + (AmountNum));
            PB2.Location = new Point(StartColumn + 87, StartRow);
            PB2.Size = new Size(447, 23);
            this.Controls.Add(PB2);
            PB2.BringToFront();
            PB2.Show();
            CheckBox CB2 = new CheckBox();
            CB2.Name = ("CheckBox" + (AmountNum));
            CB2.Location = new Point(StartColumn + 550, StartRow);
            CB2.Size = new Size(23, 23);
            this.Controls.Add(CB2);
            CB2.BringToFront();
            CB2.Show();
        }
        if (AmountNum == 3)
        {
            StartLoc = StartLoc + 1;

            //28px between each student
            StartRow = StartRow + (StartLoc * 28);
            TextBox TB = new TextBox();
            TB.Name = ("Name" + (AmountNum));
            TB.Location = new Point(StartColumn, StartRow);
            TB.Size = new Size(78, 22);
            TB.ReadOnly = true;
            TB.Text = ExcelSheet.Cells[AmountNum, 1].Text;
            this.Controls.Add(TB);
            TB.BringToFront();
            //TB.Show();
            ProgressBar PB = new ProgressBar();
            PB.Name = ("ProgressBar" + (AmountNum));
            PB.Location = new Point(StartColumn + 87, StartRow);
            PB.Size = new Size(447, 23);
            this.Controls.Add(PB);
            PB.BringToFront();
            //PB.Show();
            CheckBox CB = new CheckBox();
            CB.Name = ("CheckBox" + (AmountNum));
            CB.Location = new Point(StartColumn + 550, StartRow);
            CB.Size = new Size(23, 23);
            this.Controls.Add(CB);
            CB.BringToFront();
            //CB.Show();
         }
//The program carries on like this for all 35 potential students

Thank you in advance.