Students sitting a maths paper left the exam room to find valuables stolen from their bags.
The theft happened while 120 students were sitting a maths paper at Tendring Technology College's upper school in Frinton.
The sneak thief snatched the valuables - including keys and purses - from bags left outside the exam room.
A parent of one of the students affected said he had to change the locks on the door of their home.
The parent, who did not wish to be named, said he had to take action because whoever had taken his daughter's purse also had her college identificaton card which had her address on it.
Alan Osborn, college vice-principal and head of upper school, said he had been approached by three students following the thefts.
Mr Osborn advised students they could speak to the police individually and said the theft had been recorded in an incident book which was discussed with police.
He said sometimes pupils held on to valuables despite advice to hand them in.
Mr Osborn said: "Once the theft was reported we made it public for students. Students are always advised to hand valuables in.
"We didn't go to the police because we had not got information to take action.
"If we get information we will contact the police."
Anyone with information about the thefts is asked to contact the college or Walton police on 01255 851212 or Crimestoppers on 0800 555111
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