For循环插入多个SQL

我试图通过SQL与用户插入会话将多行数据插入ms-access。一旦我实现它keep show SQL INSERT INTO error，我已经检查了我所有的数据类型是正确的。这里需要一些帮助…

按钮:

protected void AddClassBtn_Click(object sender, EventArgs e)
{
    string clvl = DropDownList1.SelectedValue;
    DateTime Sdate = Calendar1.SelectedDate;
    int Nsession = int.Parse(TextBox1.Text);
    string Stime = DropDownList2.SelectedValue;
    string duration = DropDownList3.SelectedValue;
    string desc = TextBox2.Text;
    int teacher = int.Parse(DropDownList4.SelectedValue);
    DBconn.AddClass(clvl, Sdate, Nsession, Stime, duration, desc, teacher);

}

方法插入SQL:

public static void AddClass(string classlevel, DateTime Sdate,int Nsession,  string stime, string duration, string desc, int Tid)
{
     int no = Nsession;
     for (int i=0; i < no; i++)
     {
         OleDbConnection myconnection = GetConnection();
         DateTime d = Sdate;
         d.AddDays(i * 7).ToShortDateString();
         string mysql = "INSERT INTO Classes (ClassLevel, StartDate, StartTime, Duration, Desc, TID) VALUES('" + classlevel+"', '"+ d.ToShortDateString()+"'. '"+ stime + "', '"+ duration + "', '"+ desc + "', '"+ Tid + "')";
         OleDbCommand mycmd = new OleDbCommand(mysql, myconnection);
         try
         { 
             myconnection.Open();
             mycmd.ExecuteNonQuery();
         }
         catch
         {
             throw;
         }
         finally
         {
             myconnection.Close();
         }
     }
 }

VALUES('" + classlevel+"', '"+ d.ToShortDateString() +"'. '"+ stime + "', '"+ duration + "', '"+ desc + "', '"+ Tid + "')

string mysql = "INSERT INTO Classes (ClassLevel, StartDate, StartTime, Duration, Desc, TID) VALUES(@ClassLevel, @DateString, @Stime, @Duration, @Desc, @TID)";
OleDbCommand myCmd = new OleDbCommand(mysql, myconnection);
myCmd.Parameters.AddWithValue("@ClassLevel", classlevel);
// my advise is to not store date as string. But you should dicide for yourself!
myCmd.Parameters.AddWithValue("@DateString", d.ToShortDateString());
myCmd.Parameters.AddWithValue("@Stime", stime);
myCmd.Parameters.AddWithValue("@Duration", duration);
myCmd.Parameters.AddWithValue("@Desc", desc);
myCmd.Parameters.AddWithValue("@TID", Tid);