School uniform should be cheap and healthy, says Assembly government.
The best school uniform is low cost, practical and good for pupil health, according to guidelines for governing bodies issued last week.
Trainers should be allowed as they avoid pupils from less well-off families suffering severe foot problems in adulthood caused by ill-fitting, non-leather shoes.
But the blazer and cap should be avoided at all costs because poorer parents cannot afford them.
The document, Guidance for Governing Bodies on School Uniform and Appearance Policies, was generally welcomed by headteachers.
But some teaching unions said some makes of designer trainers were more expensive for parents than shoes and could be regarded as status symbols, contrary to the intentions of the guidelines.
The government already gives pound;97.50 to parents of pupils entitled to free school meals to buy uniforms on entering secondary school. But the extra cost of some items, including school logos, is seen as potentially divisive, says the guidance.
Wearing trainers was also seen as a way of encouraging children to "take active breaks". But schools that insist on blazers and smart shoes claimed the positive advantages of keeping up appearances made for general good behaviour.
Pauline Thomas, head of Abertillery Comprehensive in Blaenau Gwent, said the guidelines backed what the school was doing already. She said black trainers were allowed but disagreed with the blazer advice.
A few pupils set fire to a blazer in the school grounds last October after governors made them compulsory. Mrs Thomas had then sent letters out to the offending pupils' parents, but behaviour had "dramatically improved" since.
She added: "We did the right thing. Blazers aren't so expensive. You can get one for pound;26 here."
The guidelines also say if governors insist on pupils wearing a costly school logo, it should be kept to one item, as well as being sold separately to sew or stick on.
Morriston Comprehensive in Swansea banned trainers after a uniform policy crackdown last year. Peter Richards, deputy head, said the idea was to prevent the formation of gangs. But the school is flexible on religious clothing, allowing Muslim girls to continue to wear their hijab headscarves.
"Common sense and reasonableness should prevail," Mr Richards said.