Thousands of parents and children are expected to take part in a national day of action, along with teachers and governors, to protest about pressures on school funding.
Friday's events aim to highlight the issue of school cuts to all those campaigning in the general election, according to organisers Fair Funding For All Schools.
Campaign co-founder Jo Yurky said: "The significant point about this is it is people who do not normally do this sort of thing.
"This is ordinary parents picking up their children and going to do this picnic protest. It's ordinary people doing extraordinary things, people who have been moved to take action because they are so concerned about cuts to our schools."
The events taking place in the afternoon are due to include a minute's silence at 5pm in memory of the victims of the terrorist attack in Manchester.
Schools in England need to make £3 billion in savings by 2019-20, according to government estimates, the National Audit Office (NAO) has said.
The Conservatives have said that they will increase the schools budget by £4 billion by 2022, which it says represents more than a real terms increase for every year of the next parliament, although this has been questioned by the ASCL union.
Labour has said it will fund a National Education Service, with almost £5 billion extra pumped into the English school system by 2022, by raising corporation tax from its 19 per cent rate to 26 per cent by 2021-22.
The Liberal Democrats have said schools and colleges will receive a £7 billion spending boost if they win power, including £3.3 billion to protect per-pupil funding in schools.
Campaigners have also raised concerns about the effect of the government’s proposed national funding formula, which was set to see more than 9,000 schools lose money.
All three party manifestoes commit to a fair funding formula which would not see any school lose money.
At least 20 schools are expected to take part in a gathering in Brighton, at an event that will include street theatre, a children’s DJ, and a David Bowie tribute act.
In a statement, Fair Funding For All Schools said that schools are already feeling the impact of a squeeze on budgets, and that it wants to see per-pupil funding protected in real terms over the lifetime of the next parliament.
Ms Yurky added: "As a result of the brilliant campaigning by parents across the country, all parties now accept the need for more school funding.
"We've seen major concessions in the party manifestos but, sadly, not all of them go far enough.
"At the very least, we need pupil funding to be protected in real terms over the next five years.
"Anything that falls short of that is unacceptable."