The two cousins fell unconscious after they were shocked when they grabbed an iron gate at Ban Nam Dib School in Moo 8 village in Tambon Nong Pling at 3.30pm on Wednesday following heavy rains.
They were rushed to Kamphaeng Phet Hospital intensive care unit.
The hospital said the third grader, Chonlada Chat-on, 9, died at 9am on Thursday. Her blood pressure and pulse rate had been dropping since 6am and doctors resuscitated her three times but failed to bring her back.
Wanchai Kerdmeephote, deputy director of the Kamphaeng Phet Basic Education Office, visited the school to investigate the power leakage.
Thanadkijj Nuam-in, the manager of the Kamphaeng Phet electricity office, went to the school on Wednesday night.
He said they found that the school used temporary power wires at several spots that could easily cause a short circuit if the wires broke or became exposed.
Doctors said the dead girl's fifth grade cousin was now in a safe condition.