Create a class in the .css file.
a.activePage{ color: blue; border-bottom: solid; border-width: 3px;}
Nav-bar will be like:
<div id="sub-header">
<ul>
<li> <a href="Home.aspx" class= "activePage" >Home</a> </li>
<li> <a href="Admin.aspx">Contact Us</a> </li>
<li> <a href="Schools.aspx">About Us</a> </li>
</ul>
</div>