According to , detectives from the anti-graft agency visited the school on Wednesday as part of their investigation into the Governor's academic background.
In a letter dated March 28, the anti-graft agency asked the school for documents and information that will facilitate the probe into the Governor.
EACC deputy CEO Michael Mubea sought to know if he was enrolled at the school, particulars of the enrollment, if he sat KCPE there and the grades he attained.
Mubea also asked the headmaster of the Tononoka school for the Governor's leaving certificate and any other relevant information.
"With reference to your letter ... I do hereby confirm that Hassan Ali was a pupil in this school," reads a letter stamped and signed by Tom Mboya headmaster Baba Suleiman.
The letter dated April 5 goes on to state that Hassan Ali sat for his KCPE with index number 16010/19 in 1988, "thereby adopting his surname Ali Hassan".
The letter also lists the grades obtained in the exams including a C+ in English Language, B- in Mathematics and D+ in Science and Agriculture.
"He was awarded KCPE certificate number 107490," adding, "Apart from being active in football, Ali Hassan was also the school time keeper."
Joho has been under attack following a series of reports that he allegedly used fake documents to make an application to the University of Nairobi in 2009.
The claim is that Joho presented a forged 1992 examination result slip with a C+ mean grade. This was for a Bachelor of Commerce degree in Human Resource Management.
A letter from Knec purported that the county boss' KCSE papers were forged. Acting chief executive Mercy Karogo said in a letter to the DCI Ndegwa Muhoro, said they proved that the documents presented to the council were fake.
The Governor denied the claims while Serani Secondary School principal Abbas Juma confirmed he was a bona fide student at the school.
He said Joho was at the school from February 1990 and left in November 1993, having successfully sat his KCSE exams.