My knowledge here is a little rusty but from what I remember and reading on here and from a source I have had this discussion with previously, is that the tour operator normally asked for 'blocked' seating prior to departure and usually months in advance as school trips have to be planned well in advance. For an A380 I believe it was on the lower deck at the back but maybe a staffer can confirm that? I don't believe it's alphabetical and the check in staff will check folks in but they may be in chunks but very nearby usually.

Sage advice as always by CWS and no doubt once onboard the kids will sort themselves out, I've seen it happen on BA and I've been in economy quite a few times over the years. It's a fairly standard process for BA and other major airlines at this time of year with school trips etc.