The confrontation had angered Clare, and when she got inside, she found more reasons to be angry. There were several girls wearing dresses much shorter than Clare's. Clare was tall for her age. Because of that, her dress seemed shorter than it really was. On the shorter girls, their short dresses looked longer.
It was all an optical illusion, and anyone with sense should have been able to see that. Ms. Duncan's criticism of Clare's dress seemed unfair. Clare was upset.