I made an action of the workflow in this repo: https://github.com/markpatterson27/create-issues.

You can achieve the same result as the workflow based solution, using the action and a workflow with the following contents:

name: Create Issues

on:
  create:

jobs:
  create:
    name: Create project and issues
    runs-on: ubuntu-latest
    if: ${{ github.ref == 'refs/heads/feedback' }}
    steps:
      - uses: actions/checkout@v2

      - name: Create issues
        uses: markpatterson27/create-issues@v1
        with:
          github-token: ${{ secrets.GITHUB_TOKEN }}
          project-name: Assignment Tasks
          project-description: Complete the tasks outlined in each issue.
          issues-directory: .github/STARTING_ISSUES

Workflow file that prepopulates an assignment repository with issues when a student accepts the assignment.

Requirements:

• The workflow file is intended to be used with GitHub Classroom assignments.
• Feedback pull request needs to be enabled for the assignment.

Note: the workflow makes use of the Project API, which is currently in preview stage and subject to change. It is possible that by the time you read this, the API has changed and the workflow is broken.

1. Copy the .github/workflows/prepopulate-issues.yml file to your assignment repository's workflow directory.
2. For each issue you want to create, add a text file to the .github/STARTING_ISSUES directory. The filename of each file will become the issue title and the contents of the file will be the issue's body.

• The workflow runs when the feedback branch is created.
• The workflow creates a Project called "Assignment Tasks" and a column called "To Do".
• The workflow then iterates through the files in the .github/STARTING_ISSUES directory, creating an issue per file and adding that issue to the "To Do" column in the project.