“Teach 25 years, not one year 25 times” is a teaching adage of unknown authorship. A teacher should learn something new each year, and this should be reflected in the lesson plan.
 
“Twenty-five years of teaching mean twenty-five different years, not one year twenty-five times” was cited in Introduction to Education (1955) by Harold R. Bottrell.
 
   
Google Books
Introduction to Education:
Understanding Teaching Through Developmental Laboratory Experiences
By Harold R. Bottrell
Harrisburg, PA: Stackpole Co.
1955
Pg. 133:
Twenty-five years of teaching mean twenty-five different years, not one year twenty-five times, Each and every year is full of many experiences — and this is what a teacher learns, and how he grows.
     
Google Books
Advent Review and Sabbath Herald
Volume 152, Part 1
1975
Pg. 2:
Someone once commented that a certain mediocre schoolteacher had taught school 20 years. To which someone else objected, “No, he hasn’t taught 20 years; he’s taught one year 20 times.”
   
21 July 1976, Amarillo (TX) Daily Times, “Teachers Learn Uses For Papers in Class” by Nan Burk, pg. 2, col. 1:
“I know some teachers who have been teaching for 20 years,” she (Hope Shackelford—ed.) said. “But actually they’ve taught one year 20 times.”
 
Google Books
Children Held Hostage, Or, Teachers’ Organizations Demands—Are They Justifiable?
BY Phillip Carl Snyder and Earl E. Hogan
Zanesville, OH: Custom House Press
1981
Pg. 141:
In reality, many teachers who have taught 15 years have really taught one year 15 times. They teach the same thing over and over.
   
Google Books
An Introduction to Teaching:
A Handbook for Primary and Secondary School Teachers, Second Edition
Edited by Gill Nicholls
London: RoutledgeFalmer
2004
Pg. 17:
Developing an arsenal of teaching strategies, methods and styles is essential if you are to become an effective teacher. In short, the motto is: ‘Teach for 20 years; do not teach one year 20 times.’
 
Twitter
Joe Clark
‏@DrJoeClark
Great teachers teach 35 years, not one year 35 times. #tlap #edchat
8:49 PM - 6 Jun 2014
 
Google Books
101 Answers for New Teachers and Their Mentors:
Effective Teaching Tips for Daily Classroom Use,Third Edition
By Annette Breaux
New York, NY: Routledge
2015
Pg. 133:
I’ve often heard it said that some teachers teach 30 years, and others teach one year 30 times!
 
Twitter
Jennifer Marten, PhD
‏@jenmarten
@romenendez14 It’s the old adage - teach 30 years not one year 30 times. Passion keeps us growing/learning. #txeduchat
9:01 PM - 28 Jun 2015