<div id="mainnav">
<ul id="mainnav">

        <li<?php
                if (is_home())
                {
                echo " id=\"current\"";
                }?>>
                <a href="<?php bloginfo('url') ?>">Home</a>
        </li>
          <li<?php
                if (is_page('about'))
                {
                echo " id=\"current\"";
                }?>>
                <a href="<?php bloginfo('url') ?>/about">About</a>
        </li>
          <li<?php
                if (is_page('lessons'))
                {
                echo " id=\"current\"";
                }?>>
                <a href="<?php bloginfo('url') ?>/lessons">Lesson Pricing</a>
        </li>
          <li<?php
                if (is_page('student-gallery'))
                {
                echo " id=\"current\"";
                }?>>
                <a href="<?php bloginfo('url') ?>/student-gallery">Student Gallery</a>
        </li>
           <li<?php
                if (is_page('blog'))
                {
                echo " id=\"current\"";
                }?>>
                <a href="<?php bloginfo('url') ?>/blog">Blog</a>
        </li>
           <li<?php
                if (is_page('contact'))
                {
                echo " id=\"current\"";
                }?>>
                <a href="<?php bloginfo('url') ?>/contact">Contact</a>
        </li>
</ul></div>

http://wordpress.org/support/topic/how-to-menu-highlight-current-page

Wordpress - Highlight Nav

PHP