Atleast one checkbox is compulsory to be checked

Hi,
I have 5-6 checkboxes. What I want is, whenever the form is filled. I want ateast one checkbox to be checked. Here is my list of checkboxes.
<td> <div class="control-group"> <div class="controls"> <asp:CheckBox ID="chkStudent" runat="server" Text="Student" CssClass="chkClass" /> <asp:CheckBox ID="chkTeachers" runat="server" Text="Teachers" CssClass="chkClass" /> <asp:CheckBox ID="chkStaff" runat="server" Text="Staff" CssClass="chkClass" /> <asp:CheckBox ID="chkPatient" runat= ...