The risk of coronavirus transmissibility from children to adults is still unclear. Recent studies indicate that children, albeit at less risk themselves, can carry the disease for weeks. Logic dictates that they may be able to transmit. Lack of empirical proof to the contrary presents a clear risk.
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The Health and Safety at Work Regulations Act 1974 entitles all employees to workplace safety. PPE (in accordance with precedents already set in other countries) is the least school staff should expect. Other front-line workers (health workers, retail and hospitality etc) are entitled to PPE, screens and other safeguards. In lieu of reliable scientific evidence and the inability of school staff to consistently maintain social distancing in the workplace, teachers should not be an exception.