Thieves stole bikes from schoolchildren as they took part in their classes in Thornton.
Several expensive bicycles were stolen from Millfield High School, on Belvedere Road, shortly after 2pm on Wednesday.
Police have released CCTV footage of two people, seen entering the school's bike shed, who they want to speak to in relation to the crime.
Leanne Wheater's 14-year-old son was among those who had their bikes stolen.
She said cutters had been used to remove the chain on her son's bike, which cost around £500.
She said: "My son's bike was stolen, along with four or five other bikes from the bike shed. I was absolutely appalled.
"They kept my son back for an hour while they waited for the police to arrive. I wasn't contacted by the school about it; they hadn't called me to say they were going to keep my son behind so I'm quite cross about it really.
"If the children are locking their bikes up the school has a responsibility to keep them safe.
"The bike shed is at the far end of the school. If they keep the gates unlocked as well, anyone could get in."
Chris Ronan, deputy head teacher at Millfield, said: "The two people who dealt with this are not on site today and it would be wrong of me to make any comment on it until those people come back."
He said he could not comment on the thefts while a police investigation remains underway.
Anyone who recognises the two people in the CCTV footage, or has any information about the thefts, is asked to email PCSO Kirstie Jackson on email@example.com quoting reference number 0950 of the 27/02/2019.