I'm not sure about this error for education version but for commercial licenses, one serial number can be activated two times (one on office and other on home machine provided only one would one running at a time).
I've no idea how to fix you issue but check these two related links if those give you some idea to fix (please note these are for commercial licenses but could be worth checking):
Usually student editions (Student Access, a license number starting with 9020 is tied to the schools SNL license) can be activated as many times as the school has licenses in their SNL pool. So if you are using the Student Access number, to solve the problem you need to talk to your teacher. He will need to take this to the VAR and probably increase the SNL license pool.