Under health and safety law, work experience students are your employees. You treat them no differently to other young people you employ.
Your existing employers' liability insurance policy will cover work placements provided your insurer is a member of the Association of British Insurers link to external website, or Lloyds link to external website, so there is no need for you to obtain any additional employer’s liability insurance if you take on work experience students. The ABI website link to external website confirms this.
More information about managing risks is available from the HSE in Health and safety made simple.
Download Health and Safety Made Simple from HSE