To do the following you will need the System Administrator (sysadmin) role - To find guidance on finding out who has sysadmin user role in your school Click Here
Go to Admin>Personnel>Staff
Find the staff member from the dropdown box to access their profile.
Click "Create" on the left hand side under SP Logon: No
Give the staff member a Username and Password and make sure the password has lowercase, uppercase and a couple numbers so it is strong enough to save. There is a green bar that will show you what % secure the password is as you type it (E.g below)
Assign roles based on the access this staff member needs to have and make sure you save at the bottom of the page before coming away from the page.
Once the login has been set up, the staff member can go to Home > My Password to change their password to one only they know.