Parents are able to change their child's school email account password. In fact, so too can students! Here are the instructions (written for students):
1. Sign in to office.com/signin with your work or school account.
2. Go to Settings > Password.
3. Enter your old password.
4. Create a new password and confirm it. – At least 1 Upper case, 1 lower case, 1 number and 8 characters long.