In addition to the human rights and principled imperative for inclusive education, there is a powerful educational, social, and economic case to be made. Indeed, the OHCHR Thematic Study of the Rights of Persons with Disabilities (2013) has affirmed that only inclusive education can provide both quality education and social development for persons with disabilities, arguing that it is the most appropriate modality for States to guarantee universality and non-discrimination in the right to education.^[i]

• The educational case: The focus on inclusive education in individual educational planning and cooperative learning strengthens teachers’ competences. Research also highlights that supporting children with disabilities, regardless of their age,
in inclusive environments leads to an improvement in the quality of education as it becomes more person-centred and focused on achieving good learning outcomes for all children, including those with a diverse range of abilities. 
Children with disabilities, for example, have greater overall gains in academic outcomes and behaviours in inclusive environments than their peers with similar disabilities in segregated classrooms.[ii] Furthermore, when teachers are educated to include children with disabilities, the level and standard of learning for children with both with and without disabilities increases.[iii]

• The social case: Inclusive education contributes to the creation of a culture of diversity, participation and involvement into community life for persons with and without disabilities, teachers and others in the school environment as well as the wider society. Through experience of learning and playing together, all learners, together with their parents, families and caregivers, are encouraged to learn tolerance, acceptance of difference and respect for diversity, leading to eliminating stigmatization and exclusion. Inclusive education also provides learners with disabilities with greater independence, social skills, and opportunities to become productive members of their communities and exercise their rights to participate and become involved in their societies.
• The economic case: educating persons with disabilities is a positive investment, reducing poverty and exclusion from active participation in the economy. Opportunities for quality inclusive education will lead to reduced current and future dependence, and reduced caring responsibilities

[i] A/HRC/25/29 para 3

[ii]MacArthur, J. (2009). Learning Better Together: Working Towards Inclusive Education in New Zealand Schools. http:// www.ihc.org.nz/wp-content/uploads/learning-better-together.pdf; Wang, MC and Baker, ET (1985-1986). Mainstreaming programs: Design features and effects. Journal of Special Education, 19, 503-521.

[iii] Mitchell, D. (2010). Education that Fits: Review of international trends in the education of students with special educational needs. Christchurch: University of Canterbury. http://www.educationcounts.govt.nz/__data/assets/pdf_file/0016/86011/ Mitchell-Review-Final.pdf

Had you noticed this paragraph (25) in the UN General Comment on the Right to Inclusive Education?

It’s the section where they are noting what they perceive to be some of the barriers that are impeding access for persons with a disability to inclusive education(they mean mainstream or what they call ‘regular learning environments’). 
IT”S OFFICIAL- standardised testing is the wrong thing to do if you want inclusion – time for a bonfire of all the tests – the UN says they ‘must be replaced’ – how ‘official’ do you need it to be?
“Curricula must be conceived, designed and applied to meet and adjust to the requirements of every student, and providing appropriate educational responses. Standardised assessments must be replaced by flexible and multiple forms of assessments and recognition of individual progress towards broad goals that provide alternative routes for learning.”

In paras 16 they amplify this when they caution against what they call a ‘deficit’ approach:

The education of persons with disabilities too often focuses on a deficit approach, on their actual or perceived impairment and limiting opportunities to pre-defined and negative assumptions of their potential. States parties must support the creation of opportunities to build on the unique strengths and talents of each individual with a disability.

And in para 18 they caution against excluding anyone because of the degree of their impairment:

Paragraph 2 (a) prohibits the exclusion of persons with disabilities from the general education system, including any legislative or regulatory provisions that limit their inclusion on the basis of their impairment or its “degree”, such as by conditioning their inclusion “to the extent of the potential of the individual”, or by alleging a disproportionate and undue burden to evade the obligation to provide reasonable accommodation. General education means all regular learning environments and the education department. Direct exclusion would be to classify certain students as ‘non-educable’, and thereby ineligible for access to education. Non-direct exclusion would be the requirement to pass a common test as a condition for school entry without reasonable accommodations and support.

Finally this part (Para 10c)of their definition of what they mean by inclusive education is brilliant 
(inclusive education is to be understood as) …”the primary means by which persons with disabilities can lift themselves out of poverty, obtain the means to participate fully in their communities, and be safeguarded from exploitation. It is also the primary means through which to achieve inclusive societies.’

Inclusive education can be understood as:

1. a fundamental human right of all persons with disabilities.
2. a means to achieve the full realisation of the right to education and an indispensable means of realizing other human rights.[i]
3. a principle that values the well-being of all students, respects their inherent dignity and acknowledges their needs and their ability to make a contribution to society.
4. a process that necessitates a continuing and pro-active commitment to the elimination of barriers impeding the right to education, together with changes to culture, policy and practice of regular schools to accommodate all students.

General Comment no. 4

 Article 24

The right to inclusive education

UN General Comment on the Right to Inclusive Education