Officers were alerted by Farringdon Community Academy, in Allendale Road, Farringdon, Sunderland, following the incident – which is believed to have involved bottles – and have promised “robust action” against those responsible.
The disturbance followed a demonstration earlier the same afternoon against what a parents’ group have labelled as the school’s “harsh and extreme” disciplinary code.
Yet one of the organisers of the protest, which attracted around 30 adults and youngsters, said everyone who had taken part had left the school area before trouble started.
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A Northumbria Police spokesperson said: “On the afternoon of June 26, police received a report of a disturbance outside Farringdon Community Academy, in Sunderland.
“It was reported that a small protest was being held outside the school. The event mainly passed peacefully and those involved dispersed.
“Later that afternoon, police received a further report that some protesters had returned and items had been thrown at members of staff as they left the premises. Nobody was injured, but some staff were left shaken.
“An investigation is ongoing into the incident and officers will look to take robust action against those found to be involved.”
Neal Holder, the academy’s headteacher, would not comment directly on the object-throwing incident.
He did say: “It is unfortunate that around half a dozen parents decided to protest outside our academy, unsettling both students and staff.
“Since my arrival, I have always encouraged dialogue between the academy and parents/carers, in order to work together to solve any potential problems.
“Therefore, I was disappointed by the actions of a small minority.
“Over the last 18 months we have worked tirelessly, in conjunction with parents and carers, to improve standards and expectations across the academy.
“This has been fundamental in our academy moving forward.”
Demonstrators had earlier waved placards and chanted for around an hour outside the school during lesson time against what they claim is a rise in pupil exclusions and the number of children taught in isolation.
Parent Amy Pink, 38 from Doxford Park, who has had two teenagers taught at the school, said: “As far I am concerned it was a peaceful protest and everyone who was there left.
“Some of the signs were still there so I suppose people put two and two together and thought it was something to do with us.”
Miss Pink, who has worked with disengaged children, added: “Over the last two years we feel there has been an increase in the number of pupils getting excluded or being isolated in school.
“The parents started talking, realised just how much more it was happening and formed a parents’ group on Facebook.
“This has attracted nearly 400 members, led to meetings and led to the protest today.”
Ofsted rated the school as “requires improvement” in April following its latest inspection and concluded that rates of fixed-term exclusion “are too high”.
Yet its report also maintained that Mr Holder has “increased the pace of improvement” since his arrival around 18 months ago and added: “This is leading to a better quality of teaching, learning and assessment and much better behaviour.”
Mr Holder added: “During our most recent Ofsted visit in April, Ofsted highlighted that a strong disciplinary policy was brought in 18 months ago to restore behaviour and standards, improve the quality of teaching and address disruption to learning, which staff and the majority of pupils valued.
“Unfortunately, a small minority of students, and parents, have not embraced these changes and take every opportunity to challenge the Academy in its journey of improvement.
“Initially, when a new behaviour policy is introduced, students may push the boundaries to check whether its implementation is serious, therefore exclusion rates may increase as a result.
“Ofsted have acknowledged that incidents of exclusion have reduced markedly since this point, showing new behaviour expectations are beginning to be embedded.”
The police spokesperson added: “Anyone with information is asked to contact police on 496 260619 or Crimestoppers anonymously on 0800 555 111.”