Rawmarsh Community School, in Rawmarsh, near Rotherham, was forced to close at about 10am yesterday after a walkout which was staged by angry members of the National Union of Teachers.
The union claims that new headteacher Dr Stuart Wilson, who took over last September, has failed to negotiate adequately over a plan to sack 25 teachers and about 10 support staff.
NUT officials said Dr Wilson had inherited a deficit when he arrived and admitted that pupil numbers had fallen, but said staff reductions could be made without the need for redundancies.
The union's regional secretary John Dalton said: "Our action had massive support from all staff and although the school tried to stay open for year 10 and 11 pupils, it had to close at 10am.
"We are hoping that we will be able to reach some agreement with the school's management in the next few days, before two more days of action which are planned for next week."
Mr Dalton said problems started at the school in November last year, when several staff were issued with HR1 redundancy notices and told that their contracts would come to an end at Easter this year.
At that point the union threatened to take action, arguing that losing staff at that time in the year would jeopardise the performance of pupils in the 2011 summer examinations.
Redundancies have since been put back until the end of the school year to allow pupils to take their exams, but the union said it still did not accept that jobs needed to go so quickly.
Mr Dalton added: "This is an extreme number of job losses and although we agree the school needs to reorganise itself, this is not acceptable.
"The school does need to reduce overall teaching numbers but this can be done through natural wastage, slowly. Since this process started four teachers have got new jobs and have gone.
"It is not just about jobs, it is also about education of the children. There will be an impact on class sizes and this is not good news for parents."
Mr Dalton said he and his members had received "a lot of support" from parents and the wider community for their action to stop the cuts at what he described as a "good school".
Problems have arisen at Rawmarsh because the school is reportedly in debt after suffering from falling pupil numbers, an inherited deficit and the loss of a number of Government grants.
Earlier plans to tackle the deficit by previous headteacher John Lambert were opposed by the school's governing body and were not implemented.
Yesterday Dr Wilson was not available for comment, but in a letter he sent to parents he said: "Nobody wants to lose valued staff.
"However, given that we started this year with 220 fewer students than three years ago with the same number of teachers, have a growing budget deficit and are subject to the same national cuts as other schools, something had to be done.
Dr Wilson claimed in his letter that the redundancies will have no effect on class sizes, which will still be "below average".
If no agreement is reached between the school and the union, staff will walk out again next Wednesday and Thursday.