I am guessing that you mean LAX rather than LOS (LOS is Lagos in Nigeria which is also a VS destination). [:D]
You generally don't get a group discount for the reason I will mention below hence it being regarded as a group booking in the industry but I do agree it seems odd at first.
I once many years ago traveled as part of a group booking on VS (and were commented by the crew for our excellent behaviour throughout, much better than the behaviour I saw a couple of years latter from some school children on a flight back from JFK but I digress..) and we were flown in the most flexible Y Class- so the most expensive. Although we got a huge discount for other reasons. [;)]
Some of those more knowledgeable on bucket classes will be better able to answer this (que: Pete, Decker, Richard et al) but I believe you will have a cheaper Expedia price becuase it is a cheaper Economy bucket class (of which there are fewer seats available). There will not be 50 seats available in the cheapest economy bucket class hence your higher quote.
I would also imagine that VS will not allow for passengers in groups to be booked across these bucket codes. If you have a look at the
Fare Code page you will see the list of Economy bucket classes. Then look at the small print on expedia and I think you will find that the Expedia is the cheapest Economy class (B I think) and the price from VS will be a full fare economy ticket so the code will be Y.
Have you thought about using a schools tour operator such as
NST who may have a much better contract with VS in order to get a bigger discount?
As a side the same thing happened to me when organising a sports warm weather training camp to Portugal- I just got everyone to book the online cheaper fare but I guess you cant really get school kids to do that.