Police were called to Shrewsbury School yesterday after Gillian Stokes, 44, was spotted racing on to the field in the middle of a practice game at high speed.
Eyewitnesses said after coming to a halt the former tuck shop worker went "berserk" and chased a teacher around the prestigious school 's grounds with a fire extinguisher.
Staff quickly realised it was her due to the personalised number plates and called the police while leading pupils to safety from the pitch - described by cricket-writer Sir Neville Cardus as the 'The most beautiful playing fields in the world."
Kathy Campbell, a spokesman for the £30,000-a-year school - whose alumni include Charles Darwin - said: "She did did briefly and rather unexpectedly enter the school fields on Friday.
"Pupils were practising on the cricket pitch at the time when she turned up in defiance of an injunction banning her from the premises.
"She did several high speed laps of the cricket pitch in her car and performed several doughnuts.
"Because her car has personalised number plates we knew immediately who it was and alerted police.
"Teachers led the pupils to safety. Everyone was shocked but it was dealt with in a calm and swift manner.
"The police arrived quickly and she was arrested. I gather she is being held in custody. The matter is now in the hands of West Mercia Police.
"We taking extra security measures to ensure she cannot come back to the school and locking the gates.
"Obviously you don't want someone who is in an irrational state driving their car on to the school grounds where there are children around. It is very sad.
"Our groundsman is obviously not happy that someone drove a car on to the cricket pitch and the square. You are not even allowed to walk on it.
"It is quite a historic and beautiful cricket pitch. Charles Darwin probably played on it.
"We are very proud of our cricket teams at Shrewsbury and we also have some very good female players too who play in the second XI with the boys."
One parent of a pupil at the school, who did not want to be named, added: "It sounded very dangerous but I don't think she would ever hurt the children.
"It's certainly not something that goes on at a school like this. It's very worrying that she got in so easily."
A spokesman for West Mercia Police confirmed: "There was an incident at Shrewsbury School where a 44-year-old woman was arrested on suspicion of assault and criminal damage."
Ms Stokes had taken the school to an employment tribunal after being sacked in May 2013.
The Birmingham hearing was told that Stokes had worked at the school for 17 years before she was sacked after a "breakdown of trust and confidence".
It also heard that just days after she lost her job Stokes sent an email to all 750 pupils at the prestigious school claiming she had been sexually assaulted by a female member of staff.
She sued the school for £24,000 in loss of wages and "hurt feelings", claiming she was sexually assaulted.
She also claimed that, on another occasion, she was manhandled by a male member of staff.
A reserved judgement on the case was made on June 13 and although she won the hearing in Birmingham she did not receive any compensation.
Ms Stokes, from Shrewsbury, told the tribunal that she was sacked because of her activities with the UCAT trade union.
But the tribunal found that while the school did unfairly dismiss Ms Stokes, they did not breach the Trade Union and Labour relations Act in relation to her.
Stokes admitted sending the email to staff and pupils claiming she had been sexually assaulted, which she said she "regretted", adding that she had done it while suffering "great emotional distress".
However, she denied sending threatening messages to former colleagues. including one that read: "Careful when you go home. Things are bad out there. We wouldn't want you to have an accident."
See also: Fired Shrewsbury School tuck shop worker wins tribunal