I think it is desirable for secondary school students to wear casual clothes when they are at school.
Firstly, casual clothes make all students feel comfortable. They don't feel constrainted to wear uniform that they don not like.
Secondly, for one thing, casual clothes can also give students freedom of choice. They can choose what sizes, colors, and fashions of clothing they love.
Finally, casual clothes can make students feel self-confident when they are in their favorite clothes. And what's more it makes the school more colorful and lively.
In conclusion, in my opinion, all the secondary school students should wear casual clothes.