Change your WP Query arguments like this:
$args = array(
'post_type' => 'assignment',
'post_status' => 'publish',
'meta_key' => 'batch',
'meta_value' => '2017',
);
For multiple post meta you can use meta_query like this
$args = array(
'post_type' => 'assignment',
'post_status' => 'publish',
'meta_query' => array(
array(
'key' => 'color',
'value' => 'blue',
),
array(
'key' => 'year',
'value' => '2017',
)
),
);