1. Please go to Front End PM PRO > Settings > Security > Role to Role Block > Add New and set “From Role” as student role, “To Role” and teacher role and “Block For” as “New Message”. Then save changes. After this settings students will not be able to send message to teacher.

2. Add following code in your theme’s (child theme’s if you are using) functions.php.

add_filter( 'fep_get_option', function( $value, $option ){
    if( 'can-send-to-group' == $option ){
        if( in_array( 'teacher', wp_get_current_user()->roles ) ){
            $value = true;
        } else {
            $value = false;
        }
    }
    return $value;
}, 10, 2 );

Change teacher with your teacher user role. After this only teachers will be able to send group message.