DropDownList Problem Need Help and Input

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ASP.NET

Hello. I have a form that does reports. On the form I have 6 DropDownList. The main DropDownList name is Schools. The other five are years. What I want to do is to select a school from the DropDownListSchools and based on that selection the DropDownListYear will populate with the years. When I Databound the DropDownListyear it came with all the years from all the schools and no way to tell which year belongs to which school. All of the schools have an ID also. Can someone help me to see where did I go wrong? What code am I missing?

using System;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Data;
using System.Drawing;
using System.Text;
using System.Data;
using System.Collections.Generic;
using System.Linq;
using System.Data.SqlClient;
using System.Configuration;
using System.Drawing.Printing;
public partial class SchoolReports : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {

    }
    protected void DropDownListSchools_SelectedIndexChanged(object sender, EventArgs e)
    {
        SqlConnection con = new SqlConnection(System.Configuration.ConfigurationManager.ConnectionStrings["PassConnectionString"].ConnectionString);
        con.Open(); 
    }
    protected void DropDownListCFY1_SelectedIndexChanged(object sender, EventArgs e)
    {

    }
    protected void DropDownListCFY2_SelectedIndexChanged(object sender, EventArgs e)
    {

    }
    protected void DropDownListCFY3_SelectedIndexChanged(object sender, EventArgs e)
    {

    }
    protected void DropDownListCFY4_SelectedIndexChanged(object sender, EventArgs e)
    {

    }
    protected void DropDownListCFY5_SelectedIndexChanged(object sender, EventArgs e)
    {

    }
    protected void DropDownListFY1_SelectedIndexChanged(object sender, EventArgs e)
    {

    }
    protected void DropDownListFY2_SelectedIndexChanged(object sender, EventArgs e)
    {

    }
    protected void DropDownListFY3_SelectedIndexChanged(object sender, EventArgs e)
    {

    }
    protected void DropDownListFY4_SelectedIndexChanged(object sender, EventArgs e)
    {

    }
    protected void DropDownListFY5_SelectedIndexChanged(object sender, EventArgs e)
    {

    }
}