M and S are crap at measuring, but I like their bras very much. I am small of nork and find that they have a huge selection for me to choose from. I measured DD myself and bought her first bras from there as well.
Unfortunately M&S being good happens rarely. Essentially their training method is wrong (see the mega "i'm tweeting M&S" thread on S&B) so you only get a properly fitting bra in there if you have a rogue fitter who has been trained elsewhere.
I hate to say it but there is actually a very strong chance she isn't in the right size. I'm afraid the same applies for joliejolie - if you can buy your DD's bras at Primark or New Look then that suggests they probably fitted her in a bigger band than she needs as those stores don't normally do small band sizes and most teens need them in reality
I bought my dd's first bra at Marks. They will measure her as well and they are usually very helpful about it all. Now that we have her size, I have been buying bras at Primark and New Look because they have cuter styles.
She's in crop tops at the mo, but I think she needs a proper bra now. I've measured her as 27 inches under her bust and 30 inches across. does that make her a 28A? Also can anyone recommend a good shop for first bras? Thanks.