OK, I wasn't expecting such a number of responses, but I will try and respond to them as best I can.
Firstly, the tickets have not yet been purchased. The teacher has informed me today that the tickets he is looking at cost around £30, and looking at BRFares shows a ticket from our local station to London (return) priced at £32 for a child. The teacher said he thought the tickets were valid because we were still students, which may just be a mistake or may be this additional validity described in the manual which has been misinterpreted/lost in translation along the way.
I was unaware of this extended validity, and I am pleased I have discovered it. It does of course call into question the age-old problem of information being contained within the manual and nowhere else, and AFAIK none of the TOC group travel pages I have looked at give this information, so the general public are highly unlikely to ever know about it except in circumstances like this. How many ticket office/on train staff know about it I wonder?
The teacher asked me to try and find good deals on tickets for him, knowing I have good railway knowledge. When I told him of the high cost he said he would look at alternatives. After telling the class they would travel on child tickets, my friend then approached me to ask for advice, as he understood that he should not travel on a child ticket.
From the advice in the manual, is it only group tickets that have this extended validity, or all tickets if in a group of ten or more?