what are the words we can tell for the retired teacher (professor)?

Answer 1

Assuming that you want to say something on his/her last working day, and if you are his/her colleague, you could say, "Retirement is end of a job, not end of life". "Enjoy all those Sundays that you had, but couldn't enjoy till now." "There still remains a lot more to be learnt and taught." "Its about time you had taught your children something too!"
If you are a student, say, "Syllabus is covered but what about our doubts?" "How would you take an extra class at your home without a blackboard?" etc