Why I can't select the values from the combobox?

I have a project in winforms and in my RegisterStudent form I have a combobox that hold education column.Based on the selection from the combobox,a selection of checkboxes will appear.The problem is that when i want to select a value from the combobox,i can't.Instead the courses for the default value appears.This is my method to load data into the combobox:

private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)
{
    LoadEducation();
}
private void LoadEducation()
{
    using (SqlConnection conn = new SqlConnection(connstr))
    {
        try
        {
            string query = "select  education from AvailableCourses";

            da = new SqlDataAdapter(query, conn);
            conn.Open();
            ds = new DataSet();
            da.Fill(ds, "AvailableCourses");
            comboBoxEducation.DisplayMember = "education";
            comboBoxEducation.ValueMember = "education";
            comboBoxEducation.DataSource = ds.Tables["AvailableCourses"];
        }
        catch (Exception ex)
        {
            // write exception info to log or anything else
            MessageBox.Show("Error occured!");
        }
    }
}

And this is the table from where i take the data:

CREATE TABLE [dbo].[AvailableCourses](
	[CourseID] [char](10) NOT NULL,
	[ClassID] [nvarchar](50) NULL,
	[courseName] [nvarchar](50) NOT NULL,
	[education] [nvarchar](50) NOT NULL,
PRIMARY KEY CLUSTERED 
(
	[CourseID] ASC

What I have tried:

Does anybody have any idea why is not working?I have also tried with SelectedIndex and SelectedValue but for nothing,it won't do anything.Thank you in advance.

Tags: C#, MySQL