The school should be required to let you change the password or give it to you.  Since you are the guardian you have that right.  If that does not work I would try to set up a new account with the power school.  The school system is a joke so good luck I am fighting them now with getting my nephew enrolled because I have temp. guardianship with permission of my sister but I did not go though the courts.  Now they want me to just have custody which this is a temp situation.  Best of luck!!!!