Usui wears glasses when he is at home, but at school he doesn't wear them. Hence, either he is wearing contacts at school, or he doesn't really need vision aid but he likes wearing glasses at home for some reason. Why does he choose to wear glasses at home, but not at school?
Usui-kun, wears contacts at school and glasses at home. Most probably, this shows how Usui is different from school and from the comfort of his own home. Revealing the real Usui when he is out of everyone's sight. ^^