I work in health, not education, but I think that the principles are broadly the same. As I recall, the health and safety act says broadly that the employer must identify risks and minimise or mitigate them, and the employee must comply with the minimising and mitigating measures....so if a blackboard fell on your foot and you were injured because you were wearing sandals after having been warned not to, then your employer would probably not be liable for your injury but could also discipline you for failure to comply!! The school should have done a risk assessment detailing why sandals are not suitable footwear and staff should be able to see this and challenge it if it is not reasonable. There is usually a staff side health and safety rep floating around somewhere (often the same person who is the union steward) and your scholl or local authority shouls have a risk management bod who could advise.