The borough had asked its schools to make an eight per cent cut in their delegated budgets next year. But after about 1,500 parents, teachers and governors squeezed into a crucial council meeting last week councillors agreed to cut school budgets by two per cent.
However, campaigners believe that, in reality, the cuts are more likely to be four per cent because Harrow has said it will not fund the anticipated teachers' pay award at the end of January above two per cent.
It is planning total cuts of Pounds 20 million plus 300 redundancies across all services.
The authority, which appeared in the top five councils in last month's GCSE performance tables, is among the first to draw up budget plans for next year and provides an early indication of the likely scale of cuts across the country. More than 100 jobs are also at risk in Bury, Lancashire.
Earlier this month the respected Chartered Institute for Public Finance and Accountancy predicted that more than 10,000 teachers' jobs nationally could be lost as a result of the Chancellor's budget settlement for local government.
Reena Keeble, headteacher at Cannon Lane First School in Harrow, said the cuts were so serious that some heads were considering sending children home at lunchtimes.
The crisis has arisen because Harrow was given the third lowest increase among the London LEAs in its education standard spending assessment (the amount of money Government estimates councils should spend on the service).
Department of Environment figures show that Harrow received a 1.7 per cent increase in its SSA while Greenwich received 1 per cent and Lambeth -0. 2 per cent.
A statement issued by the campaigners says: "If the cuts go ahead, children will have less literacy support, a reduction in welfare support, and lower levels of midday supervision. Children will also lose out on sporting and musical opportunities."
Campaigners have calculated that a four per cent cut will mean each first and middle school could lose up to one member of staff, and secondary schools could be forced to sack up to four teachers. High schools will have to make cuts of Pounds 115,000 and middle schools Pounds 25,000.
Mrs Keeble added that Harrow was traditionally a strong Tory borough which always did well in the school league tables. Some heads suspected the borough was being penalised because it had only one grant-maintained school, she said.
More than 110 teachers in Bury could lose their jobs next year as the authority struggles to balance its budget.
Schools have been warned to expect a seven per cent cut in their budgets while council leaders attempt to make savings of Pounds 13.3m across the authority as a whole.
The authority had originally been expecting to make cuts of Pounds 11. 9m next year.
John Byrne, council leader, said: "Our education service is the envy of many and it is heart-breaking that we have to act in this way.
"We see little alternative but local people, especially parents, should be aware just where the blame lies. It is with Government."
Despite the cuts, Mr Byrne said that Bury would still be spending more on education than the Government believed was necessary.
"That shows just how out of touch Whitehall is with what is going on here in Bury. We are being asked to survive on rates which are set by the Government yet they do not have a clue just how desperate our situation is."