Click on the Staff tab of your dashboard and find the teacher who needs admin access by typing their name or email address into the search bar. Then click on the three dots next to their name and choose 'View profile'. After that click 'Manage account' and then 'Grant admin rights'.
If the person you want to make an administrator has not yet joined your school, give them the school key and ask them to sign up as a teacher and join the school using the key. You can then give them admin rights to your school account.