Looks good, but you don’t need to copy the template. Just add this function in your child theme’s functions.php file (or wherever you put the other code):
function my_theme_add_activity_tab() {
if ( !is_user_logged_in() )
return false;
?>
<li id="activity-myclass">
<a href="<?php echo site_url( BP_ACTIVITY_SLUG . '/#myclass/' ) ?>" title="<?php _e( 'Activity for my Class Year.', 'buddypress' ) ?>">
<?php printf( __( 'My Class', 'buddypress' ) ) ?>
</a>
</li>
<?php
}
add_action( 'bp_activity_type_tabs', 'my_theme_add_activity_tab' );
