Some of the following organisations may be able to help you if you are looking to do voluntary work in Africa. Most likely you will have to be self-funding and with some you must pay a placement fee.
Africa and Asia Venture www.aventure.co.uk is an organisation which takes hard working, adventurous students or graduates under 24, who want to teach in primary or secondary schools in Africa or Asia during their gap year in Kenya, Tanzania, Uganda, Malawi, Botswana, and Nepal.
Africa Inland Mission International https://aimint.org/
BUNAC offers a ‘Teach in Ghana’ programme as well as opportunities in South Africa - https://www.bunac.org/uk
Christians Abroad http://www.cabroad.org.uk/
Madventurer - A student charity that arranges summer and short expeditions to Ghana, Uganda and Kenya, including some teaching work - https://www.madventurer.com/
Peace Corps - https://www.peacecorps.gov/
Skillshare Africa - https://www.skillshare.org/
VSO - This is an international development charity that works through volunteers. The majority of positions are for a two-year period but extensions are permitted. Return flights, insurance & medical cover, visas and permits, plus a living allowance while you are overseas and housing or accommodation. Sends teachers to Eritrea, Ghana, Malawi, Mozambique, Nigeria, Rwanda and Tanzania - https://www.vsointernational.org/