Smart dinner dress for 15 yr old girl

[MerryTraveller]

There is a formal dinner coming up in the spring for the pupils and parents at my daughter's school. The only formal dinners DD has attended so far have just been with other girls, so I've been happy for her to choose a dress. These have been very tight and/or with cut out bits - very grown up, basically. Whilst this is fine with just other girls, we don't think it's appropriate for this event.
Can anyone suggest a brand selling formal wear for teens? Budget up to £150, I suppose (although would obviously prefer a lot lower!).
We live abroad so need to be able to order online.
Thank you for any suggestions!

To clarify: We live abroad but DD is at boarding school in the UK.

Monsoon teen range might be your friend. A formal jumpsuit rather than a dress.
I'm not sure tight dresses with bits cut out of them are appropriate for a formal dinner anyway, more for a dance.

Dresses for teens for school formals are a bit of a nightmare. Most stuff marketed at (and that fits!) teens tends to be a bit trashy. Good luck getting a 15 year old to wear Monsoon! You might have to embrace tight in return for her agreeing to swerve cut out bits, thigh high slashed hems etc.

Dd has worn a Lipsy dress and a Pixi Girl jumpsuit to such occasions. You might find something you can both agree on from those shops (obvs not Pixi Girl if dd is tall).

Dd has her eye on this for her next formal. I’ve no idea what the quality is like from this shop, and it’s a certain… taste…

https://www.disturbia.co.uk/collections/womens-dresses/products/slither-embroidered-blazer-dress

Thanks for the suggestions. Yes, 15 is great for casual but not so much formal... We are diplomats so I have no end of suitable clothing for any event, but it's trickier trying to find her something fun yet appropriate.