I can understand why they believe that, because they are (probably) in high school, and in that environment, there is a different perspective. In high school, you can be the star quarterback of the football team... but that doesn't mean your game is ready for the NFL. You might have knocked them all dead in your high school production of Look Homeward Angel... but that doesn't mean you're ready for Broadway.
By the same token, being a writer highly praised by your peers and teachers in high school does not mean that you've got your skills up to a professional level.
Recently I had a workshop with an editor and a major publishing house. He made a comment to a young member of our group (though he did blanket this as being somewhat applicable to all writers) about being wary of high school teacher praise. Namely, by taking said praise too seriously, you can cement bad writing ticks that served you well in high school.
I would even go so far as to argue that "good writing" in a high school context is, in fact, not necessarily good writing at all. At least from my memory, there is a certain drive at that level towards purple prose, poetical opaqueness over clarity, and a strange obsession with trying to out-thesaurus yourself, to seek synonym in order to avoid repeating the same word. That last one is exactly why you have people doing grammatical backflips to avoid using "someone said."
Things like that infected my writing well into my twenties, unfortunately.