I love school blazer 
It's really easy actually. The school have one of each size of absolutely everything. You try stuff on (took my DD1 about 20 mins max, I was there whipping stuff on and off her). You then order online, and they name tag it all. Phew. Name tagging cost £3 for my eldest child and £2 for the youngest in total.
I have had some issues with a few things out of stock this summer, but the skirt my DD needs for Friday was sent today 24 hours post and will be with us tomorrow, so actually although cutting it fine with stock she'll have everything for the first day.
You can do the online size thing, and actually it was only out for about three or four items out of the dozen or so I tried on her (and that was me wanting to go bigger to get my money's worth, so I went up a size). DD is tall and skinny. DD's blazer is currently at a local seamstress having the sleeves taken up to eek it out for the three years she's at the school.
I haven't had to send back a thing, and think the quality is excellent. So much better than Monkhouse which was the supplier before. The PE kit looks particularly high quality with proper performance fabrics.
Goodness, I do sound as if I ought to work for them 