ou tenue civile conforme à la réglementation vestimentaire

Je pense que c'est ce dont ils parlent. Les enfants doivent porter correctement leur uniforme, leur tenue de sport, et leurs vêtements personnels = ça couvre toutes leurs tenues, ils doivent être correctement habillés en tous temps. C'est ce que disent en général les règlements scolaires.

Je ne pense pas qu'il faille retranscrire ce 'smart' car on risque de faire un contresens. Il est peu probable qu'ils veuillent dire ici que si l'élève est en jeans, il n'est pas tenu de porter ce jeans "correctly and tidily". Ce ne serait pas très logique.

par opposition à décontractée / casual :

"School uniform must be worn at all evening occasions, concerts, debates, lectures, etc., unless otherwise indicated (see “Smart Change” below).

Smart Change
Smart change may be worn to performances in the Stahl Theatre, concerts and such like on occasions when announced. Smart change consists of shirt, tie, jacket and trousers, all of sober colour and leather shoes. Jeans and leather garments are not permitted. Only one set of smart change need be brought to School. All items must be named.

Summer Wear
When the Headmaster announces summer wear, blazers and pullovers need not be worn. Shirts may be worn either open at the neck or fastened and worn with a tie. Ties must always be worn with blazer or suit. T-shirts should not be worn under open-neck shirts.

Casual Wear
Casual wear may be worn after lessons, at the Housemaster’s discretion. Casual wear may be worn on Saturdays after sport and on Sundays after morning Chapel. It may be appropriate on other occasions, with the permission of the Housemaster. A certain decorum should be observed in the choice of casual wear. It should not be calculated to shock, or be scruffy or dirty. Casual wear that is deemed to be unsuitable will be confiscated. The amount of clothing brought to School should be limited. Three or four sets of casual wear would be plenty"

http://www.oundleschool.org.uk/noticeboard/docs/Clothlistboy...