(a) Except as otherwise provided in this Act, neither a local public entity nor a public employee who undertakes to supervise an activity on or the use of any public property is liable for an injury unless the local public entity or public employee is guilty of willful and wanton conduct in its supervision proximately causing such injury.
     (b) Except as otherwise provided in this Act, neither a local public entity nor a public employee is liable for an injury caused by a failure to supervise an activity on or the use of any public property unless the employee or the local public entity has a duty to provide supervision imposed by common law, statute, ordinance, code or regulation and the local public entity or public employee is guilty of willful and wanton conduct in its failure to provide supervision proximately causing such injury.