How to convert dropdown list to dropdown checklist

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I need to convert my dropdown list to a dropdown checklist, so basically the item on the list is still the same but should have checkboxes on them. Thanks in advanced.
Here's how it looks like: http://timg092191.wordpress.com/2013/08/16/ddcl/[^]
Here's the code:
From AddUser4.aspx

<asp:DropDownList ID="drpSchoolOffice" runat="server" Width="200px">
    </asp:DropDownList>

From AddUser4.aspx.cs

protected void Page_Load(object sender, EventArgs e)
    {
        if (!IsPostBack)
        {
            try
            {
                drpSchoolOffice.Items.Clear();
                dtOffice = blSchool.loadSchoolAll();
                if (dtOffice.Rows.Count > 0)
                {
                    foreach (DataRow r in dtOffice.Rows)
                    {
                        drpSchoolOffice.Items.Add(r[2].ToString());
                    }
                }
                SortedList slst = new SortedList();
                foreach (ListItem li in drpSchoolOffice.Items)
                {
                    slst.Add(li.Text, li.Value);
                }
                drpSchoolOffice.DataSource = slst;
                drpSchoolOffice.DataValueField = "Value";
                drpSchoolOffice.DataTextField = "Key";
                drpSchoolOffice.DataBind();
            catch
            {
                Response.Redirect("~/index.aspx");
            }

        }
    }