In some respects we can call the UN's education Millennium Development Goal a success. The aim was to achieve universal primary education. Between 2000 and 2015, the number of children recorded as being out of school dropped from 100 million to 57 million.

That said, although there are more children in school than ever before, there is still a global learning crisis.  Nearly a billion young people don’t have access to a proper, high quality education- the type of education that transforms lives. A recent UNESCO monitoring report showed that over 250 million children do not have basic literacy and numeracy skills even though half have attended at least four years of school.

A familiar criticism of the education MDG was its focus on quantity rather than quality.

What’s the point of an education if it’s not a good one? If you’re not learning anything in school, isn’t it better to get earning?

Last year, 29 million children in Sub-Saharan Africa thought so and dropped out of school.

It’s not just poor teaching and poor learning that leads students to drop-out. For many children, schools are violent and dangerous places. A 2014 report commissioned in Uganda by the Ministry of Education revealed that 81% of Ugandan children had experienced violence in schools. 67% of these children had been sexually abused by a male teacher. Combined with prospects that don't match the promises of learning, it’s easy to see why the dangers associated with schooling might outweigh the potential benefits and lead to school drop-out.

So, it’s welcome news that the fourth of the UN's new Sustainable Development Goals (SDG 4) addresses both education quality and school environment as well as enrolment. It also includes a whole lot more. In fact, SDG 4 is remarkably ambitious. If the goal can be achieved, all children (however marginalised) will access high quality education from early childhood up to tertiary level and will have meaningful technical and vocational pathways.

Whilst the ambition of SDG 4 is to be commended, there is a concern about how it will be funded. The education agenda alone seems daunting and that’s before we consider the ambition of the other 16 SDGs. (Take SDG 1, for example, to “end poverty in all its forms everywhere”). Add to those the pressing humanitarian emergencies of our time - Europe’s refugee crisis, the Syrian civil war, ethnic and religious violence in the Central African Republic and a raft of natural disasters, and the pot of donor aid becomes woefully inadequate.

Whilst it’s difficult to argue for a hierarchy of SDGs, it's easy to make a strong case for investing more than the current 2% of yearly international aid on education. Surely an educated global population is more likely to stem climate change, achieve gender equality and live healthier lives than an uneducated or partially educated one?

Education underpins the whole sustainable development agenda.

That's why we need to do better at finding the evidence to make the economic case for investing more in education. We need to get behind organisations like  the International Commission on Education Financing that is demonstrating how education can lead to greater economic growth, better health outcomes, and improved global security. And,as outlined in the UN's Addis Ababa Action Agenda (AAAA), we need to recognise that we'll need public and private and charitable funds to achieve the 2030 sustainable development agenda.

When it comes to education, the private sector doesn’t seem to need much encouragement. In his seminal work, ‘The Beautiful Tree’, Professor James Tooley described the mushrooming of low cost private schools (LCPS) across the developing world. The LCPS phenomenon represents a market-response to the challenges of access and quality in education. In many slum areas where children cannot access high quality education, local entrepreneurs have responded by setting up a school to meet their needs and charge affordable fees for the service. Win-win surely?

Not for everyone. For some, the growth of LCPS networks is a cause for concern; that there is an inherent contradiction between profit and the human right underpinning access to quality education. Critics argue that, although LCPSs can expand access to quality education, they don’t expand access equitably to the very poorest or to children found in unprofitable locations such as remote communities or conflict areas.

But if we want to meet the rather daunting education goal of SDG-4, we’re going to have to push ideological differences aside and accept that national governments probably won’t be able to fund and deliver all the education to all the children without the support of the private and third sectors.  Equally if we are to embrace a pluralistic system we need to make sure that it is well-regulated, well-coordinated and insists on a culture of accountability. At the moment, there are many places in the developing world where it is none of these things.

The profit motive can be a source of innovation that improves standards, but it also leads to competition that may not always be atuned with what's best for education. With a greater proportion of education delivered privately, it is imperative that there are strong policy and regulatory systems in place.  Governments needn’t deliver all education itself, but they certainly needs to supervise it and foster the spirit of collaboration. After all, we’re only going to meet SDG4 if we’re all pulling in the same direction.

Laura Brown is the Head of Education at PEAS (Promoting Equality in African Schools) a UK charity and social enterprise hybrid that delivers equal access to affordable, quality and sustainable secondary education across Africa.