The Welsh Government has just published an update for schools on Covid 19. Learners are no longer obliged to test themselves. If they have symptoms and are ill, like any other virus, they are advised to stay at home from school until they feel better.
However, if they choose to test and the result is positive, they are should self-isolate for 5 days minimum. They should then test again twice, to get two negative results before they return to school. On the tenth day they can return to school regardless of the result.
WG Update: Guidance for people with symptoms of a respiratory infection, including COVID-19 | GOV.WALES