In the United States, as well as most of the world, copyright in a work is given either to the creator of the work or to their employer. Since your school is not considered your legal employer, that means any essay, painting, photograph, song, or other creative work you make as part of your academic career is owned by you.
This is true pretty much universally, including if you’re a minor or even if you use school equipment. The works you create for the classroom are yours and you alone hold the copyright to them.
With that copyright comes a series of rights to the work. Those rights include:
- The right to reproduce or copy a work
- The right to create derivative works
- The right to distribute the work
- The right to publicly perform the work
- The right to publicly display the work
These are your exclusive rights in the work and only you can authorize others to do these things with your creations.
That being said, when you submit a work to your instructor, you are granting them a narrow license to use the work in conjunction with the assignment. For example, if the assignment calls for the work to be displayed in the classroom or to the school, then you are agreeing to that by submitting the work for that assignment.
Similarly, if you are required to submit the assignment to a plagiarism detection service, you will also grant that service a limited license related to the work for the purpose of checking it for plagiarism and matching future plagiarism checks against it.
In short, the service will check your work for plagiarism and retain a copy to match against future works, but that doesn’t amount to any transfer of ownership.
You, as the creator, are the copyright holder and are still free to copy, share, and license the work to others-- or even sell it. Basically, you control the work and can do with it as you please.
This is an important thing to remember as you go through your academic career. Though you are a student, you’re creating works that you can build on and even profit from later. That short story you write for for a class could become a novel or even a movie later. An essay you write in high school could become the basis for a dissertation or thesis in college.
The work you’re creating now isn’t just useful for passing a class or getting a good grade on an assignment, they can be the building blocks that you use to create amazing new works in the future.
That’s possible because, through copyright, you own your creations and, though you license it for use in an assignment, you still control that work and its fate.
Where that work goes outside the classroom is up to you...