Hi Lib
First of all layers, layers, layers as she'll be warm/cold.
Then layers because they can help disguise any prominent breasts. She needn't worry that they'll be the first thing some people zoom in on. Also, no frills, ruffles and fussy details over the chest area. Look for a good bra too as so many teen/High St ones are ridiculously padded making breasts look like weapons.
A long line tank layered over another one in a different colour w/ a modern looking cardigan or jacket over looks smooth and streamlined. Nothing that billows out over the chest either as the material tends to hang straight down from the widest, sticking out bits making you look larger. The standard good quality blazer jacket or even a fitted bomber would look good. A long slouchy suit jacket, a casual one is an adaptable item for her to wear w/ so many different outfits.
So think fitted underneath, layered over the top and a good bra.
If she sees any new season sweaters look for ones w/ a fitted cut to the waist and a deep welt (knitted bit that settles on the hip) as that'll define her shape without looking sacklike. The new sweatshirts could look fine but make sure they are not long in the body, but fitted to the lower waist and not too sloppy.
A stretch longer skirt would work too allowing her to crawl about under desks etc. An A line shape worn with a pretty but simple shirt and some shoe boots is good office wear. Look for colour and print to keep it young. Or go long skater shape to the knee worn w/ thin cotton tights if it sin't too hot. Probably best to go w/ pants though. Geometric print straight leg pants worn w/ layered tanks and a jacket. Or a fitted shirt or blouse w/ give under the arms and not too short in the back. Get her to stretch in it and bend over to check that there's no ride up.
Look for layering in different shades of the same such as a pale grey and dark grey tank together or top/jacket/ as that looks very pulled together.
Finally motif tops can help disguise breasts. Not really small prints but the animal photo or graphic ones will draw the eye to the print not the breasts. Keep the print large but in one place rather than all over the top. No spotty tops. They tend to emphasise in my opinion.
Have a lovely day!