Because “gokurõ-sama desu” (“thank you for your hard work”) is considered to be an expression of gratitude spoken from someone higher in status to someone lower, it is said that it is best not to use it towards someone of higher status. However, it’s also not particularly appropriate for students to say “Otsukare-sama desu” (Thank you for your hard work”) to their teachers, so care should be taken.— Jreibun