Usually, we reserve changing passwords for accounts that have already submitted an application. Until an application has been submitted, an account is not technically tied to any school, so no school "owns" that account.
However, if you do find your school in this situation, the first option is to have the parent reset their own password using the "Forgot password" button on the Sign-Up page.
Secondly, you can "create an application" on behalf of the parent, change their password on the student profile, and then simply cancel the application afterward if needed.
Thirdly, in extraordinary circumstances, you can contact your SchoolMint representative and ask them to change the password for you. Please note: you will need the account's username when sending the request.