The Kenya National Examination Council is watching 100 schools to curb cheating in the KCSE exam.
Chairman George Magoha told the Star on the phone on Thursday bribery, corruption and use of phones are fueling exam cheating.
In an earlier directive, Knec allowed only headteachers to have phones in schools and not in exam rooms.
The police were supposed to monitor headteachers' phones.
Magoha said this has been difficult because it is not an easy
task for the police to track all the phones in exam centres.
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Invigilators' phones are to be locked away before an exam begins.
Magoha said headteachers and parents bribe invigilators and some security officers to have exam papers opened earlier and leaked to students.
He said headteachers then send exam questions to other centre managers who leak them to their students.
Forty people have been arrested over attempts to leak the exam.
They include invigilators, supervisors, police and students. Four candidates have been deregistered.
Magoha said exam papers should only be opened on the scheduled time as indicated in the timetables.
"Most cases under investigation involve school heads photocopying exam materials and helping students solve the questions," he said.
The exam which entered the 16th will be completed on November 30.