I saw the phrase "80 Years in Excellence" written as part of a school's advertisement. I believe it should be written as 80 Years of Excellence. Am I correct? Also, I am not sure if the number should be written as Eighty or 80?
In advertisements and newspaper headlines, it's quite common to fine a number written as digits rather than as a word, so no problem with "80 Years of Excellence"; it's fine. In formal writing, it's usually preferable to write it as a word "Eighty Years of Excellence".