"Welcome Back, Kotter" was about a dedicated teacher who wanted to return to his alma mater to try to deal with a bunch of remedial, misfit high school students in inner city NYC when no one else wanted to deal with them. These types of teenagers were not tackled on TV before. The casting was perfect for the NYC setting: from the nerd in Horshack, to the cool maverick in Barbarino, to the Latino in Epstein, to the Black male, of course, in Washington. There is also the Principal in Mr. Woodman. The writing was great. The timing was awesome. The theme song by John Sebastian is breathtaking. The show was purely magical in its first few seasons.
There were problems, as life deals us sometimes. One was Marcia Straussman. She was very unhappy that her involvement in storylines was limited. It was unfortunate because the show primarily dealt with life at the school. Because she played the wife of the teacher, and she was primarily at home, there was not room for her. The act of making her a character on the show was not a good one. The Mrs. Kotter character would have been more appropriate on recurring basis. Another problem was differences between Gabe Kaplan and the other producers and writers. This explains why we never saw him much during the later run of the series.
Gabe Kaplan's lack of involvement in the show's fourth and final season was just one of the many problems which doomed the show. The writing in that final season was sloppy, unrealistic, unfunny, and was so amateurish. As a teenager watching the show in reruns, I saw that something was amiss. The actors on the show complained that the scripts were trash. A storyline about Horshack getting married was about as bad as the writing could get, and it was that. The E! Channel's "E! True Hollywood Story" about this show talks about that dismal fourth season. Another major problem with that show in the fourth season was that the actors who played the Sweathogs. The problem with actors playing teenagers is that they were older than teenagers when they began portraying those characters. To prepare to portray teens, they had to learn how to be teenagers again. It worked in the early days.
However, by the time the fourth season had arrived, the actors had matured and developed as adults where they were getting too old to portray teenagers anymore. They also did not look like teenagers, either. Let's not forget John Travolta and his blossoming as a movie star. These factors led to the demise of the series.
The series was about a concept so fresh, people in this modern era can relate to it even more now than they could back in the 70's. This concept is about misfit children. This is why it was so popular for awhile in syndication. However, it fizzled in syndication because when those fourth season episodes began airing, the viewing felt that the whole show was crap and stopped watching. USA Network had it. TV Land had it. They both stopped showing it.
Even though things did not end on a good note, true fans of the show can ignore that fourth season and remember the greater moments. It was a great show in general.