All Muslim women should try their best to stick to the Islamic dress code in public and to take special consideration of their outward appearance when entering a mosque. In addition, every Muslim is required by Islam to perform daily prayers on time.
The Islamic Religious Council of Singapore states the following: Performing prayer on time has higher priority than being worried because of wearing a skirt such as school uniform. Thus, women should strive to perform your prayer on time, even though it means they have to enter a mosque wearing a school uniform. What we can advise you is to carry a long light-weight skirt and put it on over the uniform in the mosque. And if they can bring along something like a long-sleeved sweater and a tudung (hijab) that will be much better. Or better, bring a long head covering that covers the arms. But if that is impossible, they may approach the mosque attendant and explain to him their situation that they will be performing their prayer there on a regular basis. We are sure the attendant will allow them to keep prayer clothes there that will cover their necessary `awrah for salah.