One well-established principle that can be applied to writing processes is that variation is part of any process. No one writes consistently well all of the time. Some people call it inspiration. One problem with developing writers is that they too often wait for inspiration to begin writing. If all writers waited for the muse to inspire them, few words would ever be written.
Inspiration is a very small, even miniscule part of writing.
Writing takes work, not just inspiration, and that work is often uneven because writing is a process and variation is part of any process. The reality of variation in processes is one reason it is very important to give a writing task time. If you don't allow enough time, the emphasis is then on production, not on process, and the quality of the product always suffers when the emphasis is on production instead of process. There is a significant difference between learning to write and writing to learn. Grades and whether you are the best writer in the class matter less than what you learn through your writing process.