If you already have a pre-existing account (e.g., if you have another child who attends OWNCS), you DO NOT have to create a new account, but simply add your additional children to that current account. Simply sign in to your account, and go to account preferences in the sidebar.
Account preferences > Student tab > "Add" button (upper right corner)
From there, for each additional child, fill in the requested information.
To create a parent account, you will need to enter the following information: