Here is what we typically suggest schools do to handle this.
You will need to be a site admin to do this.
Change the year to be the one in which you want to add the students (typically the one following the current year) and go through the first few steps of the Promote process (Admin/Promote), through the step where you have setup the homerooms for next year (but have not added any students to them).
Then you can leave the Promote process. You will be able to return to the Promote process as frequently as you want until you are ready to “turn on” the 2018-2019 school year.
Go to the Classooms page and add a new classroom, select the grade, and call the classroom “Unnassigned”
You can then add students to the homeroom/classroom. Once you know what real homeroom the students will be in, you can use the ‘Move Homeroom’ button beside each student to place them in the appropriate homeroom. Once all of the students have been assigned, you can delete that ‘Unassigned’ homeroom.