The sequence you describe hasn't happened for several years. The Cotswold trains have for some time now been pathed to run into Oxford behind the stoppers, which should, in theory, be fine and gives good connections to the Cotswold Line from Thames Valley stations.
But if the stopper is late, or the despatching of the empty stock to the sidings at Oxford (or a stopper going on to Banbury) is less than sparkling (which is what happens all too often) then it is a problem, with the Cotswold Line services losing time held outside the station waiting for platform 2 to become available.
In the specific case of the 08.22, it can get held up at Oxford by the preceding 07.27 from Paddington and it is then due to meet the 08.26 from Worcester Foregate Street to Paddington at Charlbury, at the end of the single line section from Wolvercot junction, in what is a very tightly-timed meet, where every minute of delay on the 08.22 pretty much immediately puts a minute of delay on to the 08.26. And unless the 08.22 is more than 15 minutes late, it usually gets priority on the single line, thus badly delaying the 08.26 as well.
So if loading the group of schoolchildren and their luggage a few minutes before other passengers helped get that 08.22 out of Paddington on time and into Charlbury on time (and with all the cases they were carrying, it looked like something needed to be done about them), then the member of staff concerned did absolutely the right thing. She wasn't, as suggested above, over-dramatising anything, she was seeing the bigger picture, which some posters above might try to do.