I think to give the height in px is not a good prectice.
please try to give it in "Percentage" it will surly solve your problem
<table width="100%">
<tr style="height: 30%">
<td>
<asp:GridView ID="gvwSch"runat="server"
AllowPaging="true" Width="100%"
PageSize="5">
</asp:GridView>
</td>
</tr>
</table>
THen another good solution for you is to make the grid scrollable after the specified records are excedded.use the
tag and make it scrollable.
this will give you better look on the screen.