Office365/Azure or Google at YOURSCHOOL-dashboard.i-gtm.com.
NOTE: Staff login is always at YOURSCHOOL-dashboard, whereas parent login is just YOURSCHOOL.i-gtm.com (where YOURSCHOOL is the name of your school set up in I-GTM and advised to you by your administrator). All web addresses are DOT I-GTM.com (xxxx.i-gtm.com).
The majority of schools use Microsoft Azure to authenticate staff.
The staff login screen has just Google and Microsoft buttons.