Hi,
By default RadioButtonList will give -1 index only, so you want to give by default one value should be selected like below.
<asp:TemplateField HeaderText="Attendence">
<ItemTemplate>
<asp:RadioButtonList ID="rbAtt" runat="server">
<asp:ListItem Text="Present" Selected="True" Value="p"></asp:ListItem>
<asp:ListItem Text="Absent" Value="A"></asp:ListItem>
<asp:ListItem Text="Leave" Value="L"></asp:ListItem>
</asp:RadioButtonList>
</ItemTemplate>
</asp:TemplateField>