The sub password is set in the admin section of Powerschool here: Start Page > School Setup > Substitute Sign In Settings (see below)
If the school clicks on the include date checkbox then the password will change day by day to include the number of day in the month that it is. So if today is the 20th and the password they set is Sub55Login then the full password will be 20Sub55Login (note the 20 tagged at the beginingbeginning). This is better as it somewhat changes the password day to day although its predictable.