Two men, whose faces were reportedly covered with Balaclavas or hoodies, made off with the bikes from St Hild’s Church of England School in the middle of the day.
They are said to have arrived in a van and used bolt cutters to steal the cycles which had been locked in a bike shed.
The parent of one of the victim’s expressed that people could gain unauthorised access to the school site to commit the crime.
Vicky Sedgwick, mum of 14-year-old Ellis Sedgwick, whose £300 Carerra mountain bike was stolen, said: “The fact that two men in Balaclavas and bolt cutters have managed to get onto the school grounds and help themselves to two bikes is quite scary.”
Ellis got the grey and green coloured bike for Christmas and used it to get to and from school.
But St Hild’s insists regular checks including site access are carried out to keep pupils safe.
Headteacher Tracey Gibson said: “We take the safety of our pupils and staff very seriously at St Hild’s.
"Regular safeguarding checks, which include site access, are carried out.
"I am confident that our school building and pupil outside areas are secure, and pupils and staff are safe.”
A sign warning bike owners that they leave their cycles in the school bike sheds at their own risks has been put up at St Hild’s.
The thieves struck at around 1.25pm on Wednesday, April 27.
A Cleveland Police spokesperson said: “Police received a report of theft of two black mountain bikes from the school at around 1.25pm on Wednesday.
"It is believed that two men wearing hoodies arrived in a blue vehicle and used bolt croppers to get the bikes. They are then believed to have ridden off on the bikes towards King Oswy.
“Inquiries are ongoing and police would appeal for any witnesses, dash cam or CCTV owners or anyone who may have seen the bikes offered for sale to get in touch."
Anyone with information is asked to contact police on 101, quoting Ref 070026.
Alternatively, contact Crimestoppers, anonymously, online at www.crimestoppers-uk.org or phone 0800 555111.