一个对我算难的问题

页面上一个GRIDVIEW，显示学生各学科成绩。启用了自带的分页属性。在页脚显示每一科的平均分，第一页正常，但是第2页页脚显示结果是是整个GRIDVIEW数据列的和除以第2页所包含的行数，结果明显偏大。我想前面几页不显示平均数，只在最后一页的页脚显示。怎么做，谢谢大家了。我的代码是：
protected void Page_Load(object sender, EventArgs e)
    {
        string conStr = "Data Source=localhost; Integrated Security=SSPI; Initial Catalog=student;";
        string sqlStr = "select * from studentTable";
        SqlConnection sqlCon = new SqlConnection(conStr);
        SqlDataAdapter sqlDa =new SqlDataAdapter(sqlStr,sqlCon);
        DataSet sqlDs = new DataSet();
        sqlCon.Open();
        sqlDa.Fill(sqlDs,"studentTable");
        .DataSource = sqlDs;
        .DataBind();
        sqlCon.Close();
    }
    int englishSum = 0;
    int mathSum = 0;
    int politicalSum = 0;
    int chineseSum = 0;
    int physicsSum = 0;
    int historySum = 0;
    int biologySum = 0;
    protected void _RowDataBound(object sender, GridViewRowEventArgs e)
    {       
        if (e.Row.RowIndex>=0)
        {
           englishSum += Convert.ToInt32(e.Row.Cells[4].Text);
           mathSum += Convert.ToInt32(e.Row.Cells[5].Text);
           politicalSum += Convert.ToInt32(e.Row.Cells[6].Text);
           chineseSum += Convert.ToInt32(e.Row.Cells[7].Text);
           physicsSum += Convert.ToInt32(e.Row.Cells[8].Text);
           historySum += Convert.ToInt32(e.Row.Cells[9].Text);
           biologySum += Convert.ToInt32(e.Row.Cells[10].Text);
        }
        else if (e.Row.RowType == DataControlRowType.Footer)
        {
            e.Row.Cells[4].Text = "AVG:" + ((int)(englishSum/.Rows.Count)).ToString();
            e.Row.Cells[5].Text = "AVG:" + ((int)(mathSum / .Rows.Count)).ToString();
            e.Row.Cells[6].Text = "AVG:" + ((int)(politicalSum / .Rows.Count)).ToString();
            e.Row.Cells[7].Text = "AVG:" + ((int)(chineseSum / .Rows.Count)).ToString();
            e.Row.Cells[8].Text = "AVG:" + ((int)(physicsSum / .Rows.Count)).ToString();
            e.Row.Cells[9].Text = "AVG:" + ((int)(historySum / .Rows.Count)).ToString();
            e.Row.Cells[10].Text = "AVG:" + ((int)(biologySum / .Rows.Count)).ToString();
        }
    }
    protected void _PageIndexChanging(object sender, GridViewPageEventArgs e)
    {
        .PageIndex = e.NewPageIndex;
        .DataBind();
    }