Mr. Holland's Opus

In 1965, Glenn Holland (Richard Dreyfuss) is a professional musician and composer who has been relatively successful in the exhausting life of a musician. However, in an attempt to enjoy more free time with his young wife, Iris (Glenne Headly), and to enable him to compose a piece of orchestral music, the 30-year-old Holland accepts a high school teaching position.
Unfortunately for Holland, he is soon forced to realize that his position as a music teacher makes him a marginalized figure in the faculty's hierarchy. Many of his colleagues, and some in the school's administration, including the school's principal (Olympia Dukakis) and vice principal (William H. Macy), resent Holland and question the value and importance of music education given the school's strained budget. However, he quickly begins to win many of his colleagues over. In the classroom, Holland finds success utilizing rock and roll as a way to make classical music more accessible to his students. Gradually, stubbornly, he begins to see his students as individuals and begins finding ways to help them excel.
When Iris becomes pregnant, Holland uses the money saved up to produce his orchestration in order to buy a house. Their son Cole is born during the summer following his first year as a teacher. When school resumes, Holland is put in charge of the school marching band. The school's football coach Bill (Jay Thomas) helps him in exchange for allowing an academically challenged football player/wrestler named Louis Russ (Terrence Howard) to play the drums for academic credit so he can keep his spot on the wrestling team.
At home, Holland's lack of quality time with his wife creates tension. When their son Cole is diagnosed as deaf, Holland reacts with hostility to the news that he can never teach the joys of music to his own child. His wife willingly learns American Sign Language to communicate with their son. However, Holland, acting out of frustration and resentment, learns at a much slower rate, causing further estrangement within the family.
Over three decades, Holland forms closer relationships with his students at John F. Kennedy High School than with his own son. At one point in the film, he is briefly tempted by the shining singing talent and beauty of Rowena Morgan (Jean Louisa Kelly), who invites him to leave his stressful, unsatisfying life and run off to New York City with her. While greatly attracted to Rowena, Holland decides that he must stay with his family.
In a conversation with Cole, about the recent murder of John Lennon in 1980, Holland coldly claims that Cole, being deaf, can never understand what music means to him. Cole lashes out and reveals that he does appreciate music but needs his father to reach out to him. The incident encourages Holland to find different ways for Cole and other deaf children to experience music, and he puts on a concert for them during which he sings and signs John Lennon's Beautiful Boy, directing the song towards Cole.
Holland addresses a series of challenges created by people who are either skeptical of, or hostile towards, the idea of musical excellence within the walls of the average middle-class American high school. Holland inspires and touches the lives of many of his students but always seems to have too little time for himself or his family, forever delaying the composition of his own orchestral work. Ultimately, he reaches an age when it is too late to realistically find financial backing or ever have the composition performed.
In 1995, the adversaries of the Kennedy High music program win a decisive institutional victory. The school's former vice principal, now promoted to principal, works with the school board to eliminate funding for the music program along with other fine arts programs, thereby leading to Holland's early retirement at the age of 60. Holland protests to the school board that eliminating the arts will have negative impact on students, but is overruled. Holland realizes that his career in music is likely over. He believes that his former students have mostly forgotten him and is dejected at his failure to ever have his composition, which he views as his life's work, performed.
On his final day as a teacher, Iris and Cole, now an adult and a teacher himself, arrive to help Holland pack up. Because of his disappointment over his self-perceived lack of achievement, in a prearranged plan, Iris and Cole lead Holland to the school auditorium where he is surprised to find a gathering of many of his former students, now grown. Hearing that their beloved teacher is retiring, the students have secretly returned to the school to celebrate his career.
Holland's orchestral piece, never before played, has been distributed to the students by Iris and Cole. One of his most musically challenged students, now Governor of Oregon, sits with her clarinet. The students ask their former teacher to serve as conductor for the premiere performance of Mr. Holland's Opus ("The American Symphony"). As Holland conducts the orchestra of his former students, a proud Iris and Cole look on, appreciating the affection and respect that Holland receives. Holland, overwhelmed with emotion, finally realizes his masterpiece.

Question: How long does Holland work at the school?
Answer: over three decades; over three decades