WordPress: Shortcode Loop with taxanomy

wp_shortcode_loop_cpt_tax.php

function sc_loop_highlights_lehrer() {
	$args = array( 'post_type' => 'lehrer', 'posts_per_page' => -1, 'orderby' => 'title', 'order' => 'ASC', 'post_status' => array('publish'),
	    'tax_query' => array( 
	        array( 
	            'taxonomy' => 'highlights',
	            'field' => 'slug',
	            'terms' => array('ja')
	        )
	    )
	);

    $string = '';
    $count = 0;
	
    $query = new WP_Query( $args );
    
    if( $query->have_posts() ){
        
        $string .= '<div class="list-posts">';
        $string .= '<div class="lehrer-referenten-highlights wrap-cpt cf">';
		$string .= '<h2 class="heading-title">Highlights 2017</h2>';
        
        while( $query->have_posts() ){
            $query->the_post(); 
            $count++;  		
            global $post;	
           	        
			$string .= '<div class="cpt nth-'. ($xyz++%2) .' ">';
			/*	$string .= '<a class="block scroll " title="' . get_the_title() . '" href="' . esc_url( get_permalink(3970) ) .'#lehrer-referenten-id-' . get_the_ID() . '">'; */
			$string .= '<a class="block" title="' . get_the_title() . '" href="' . esc_url( get_permalink(3970) ) .'#' . $post->post_name . '">';
					
					$string .= '<div class="outer cf">';
					if( has_post_thumbnail() ) {  
						$string .= get_the_post_thumbnail( $post_id, 'partnerlogo' );
					}
						$string .= '<div class="inner">';
							$string .= '<h4>'; 
								$string .= get_the_title();
							$string .= '</h4>';
						$string .= '</div>'; 
				
					$string .= '</div>';
					
				$string .= '</a>';
			$string .= '</div>';						                    

        }
        $string .= '</div></div>';
    }
    wp_reset_postdata();
    return $string;
}
add_shortcode( 'highlights_lehrer', 'sc_loop_highlights_lehrer' );