Can't log into Power Teacher/Gradebook

These are recommendations from Pearson Tech support
Clear cookies and internet history and re-start the machine

Thank you
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The most likely cause is that the teacher did not logout of the prior session, rather closed the window without logging out.
TSGs suggestions are valid. Powerschool makes extensive use of cookies, and if a user does not clear these from time to time, they may be seeing issues re-negotiating the connection to PS through the browser.
To clear the cookies and cache and alter the settings on the browsers: Chrome:
https://support.google.com/chrome/answer/95582?hl=en
Internet Explorer:
https://support.microsoft.com/en-us/kb/263070
Firefox
https://support.mozilla.org/en-US/kb/how-clear-firefox-cache
Safari:
https://discussions.apple.com/thread/4160058?tstart=0