I cannot answer specifically about child seats but my sister runs a number of out of school clubs and an adjacent minibus transport business for schools. I do know that the regulation is extremely tight and they get checked up on very regularly (quite rightly - as she would agree).
I am absolutely certain the regulations would not let children be put in the position of using inappropriate seats or restraints.

If the transport is a Taxi, then no additional restraint is required.

If the transport is a minibus, then again no additional restraint is required. However, if a MPV or car is being used and it is not a licenced taxi, then suitable child restraints must be provided.

http://www.direct.gov.uk/en/TravelAndTransport/Highwaycode/DG_069855

By law, seat belts must be worn by drivers and passengers at all times. It is the responsibility of the driver to ensure that all passengers comply with this regulation. New seat belt laws require children under 135cm to wear child restraints in addition to seat belts. Driver responsibilities will vary depending on the type of minibus driven.

Ignorance to these seat belt laws will result in a new offence “Failing to ensure the safety of a passenger entering, alighting or conveyed in a vehicle” being committed, especially introduced for drivers of buses, coaches and minibuses. Drivers failing their passenger responsibilities can either expect to receive a £30 fixed penalty notice or a maximum fine of up to £500 if the case goes to court. Minibus owners and operators could also be fined a maximum of £2,500.

Source: http://www.structured.co.uk/minibus/school-minibus.html

The Law makes specific provision for public buses on the grounds that the vehicle operators can't know what age of passenger they will be carrying. Children in such cases must wear the adult seat belts. There is little excuse for a minibus or bus being used specifically to transport shool children though.

three of my kids are picked up every morning by their school mini bus,there are no car seats or anything like that,but there is an escort on the bus who checks all of the children are sitting properly and that all seat belts are correctly fastened.
my youngest is still quite little for his age and i noticed that the seat belt was cutting him accross the neck a little so i went and got one of those straps you can put on the seat belt so that it sits a little bit lower down and the escort puts it over his seat belt every morning.
i personally find it rediculous that these companies dont have to provide proper seating for children because if i were taking my kids to school by car then they would need the proper seating equipment,i would have thought that there was just as much risk of crashing in a bus as there was a car.
hope this helped,
take care