You don't need to check your insurance, the school does.
I can't find the national guidelines online but this says they're not too different.
If a school wants to transport pupils in private cars for a school trip they must:
Ensure that the driver is not alone with any children. These are for safeguarding reasons but also because a driver cannot supervise children and drive.
Ensure the driver understands their legal obligations when transporting pupils.
Evidence that drivers (teachers and parents) have a clean and valid driving license without points for speeding, drink/drug driving, talking on a mobile whilst driving, careless or dangerous driving.
A copy of the driver’s license should be taken and kept on file and reviewed every three months.
Evidence that adequate insurance cover is in place.
Evidence that the car is road worthy by taking copies of the MOT certificate, records of how often the car is serviced, checking tyres are safe and correctly inflated.
Evidence that drivers (teachers and parents) are medically fit to drive and that they have had eye tests to determine if driving glasses are required.
Ensure that each driver has knows how to get to the destination and not drive in convoy.
Ensure that each driver is equipped with a mobile telephone for emergencies.
Ensure that drivers are given an emergency plan to follow in the event of breakdown or road traffic collision.
Ensure that drivers understand and are familiar with seat belt legislation and that all passengers will be secured properly.
Ensure that all children will be provided with child or booster seats if they are under 12 years of age or under 135cms in height, whichever they reach first.
Ensure that no child is carried in a seat with or without a child or booster seat in a lap belt.
Ensure that no child is carried in the front seat of a vehicle where an active airbag is fitted.
Ensure that specific written permission is given by each parent for their child to be transported in private cars.