My beloved 3 years old niece was seen crossing the road outside the school alone after school ended and one of the student's father was alerted by a lady at the roadside to save her. He then returned her to the school. My sister was only 10 minutes late to fetch her and this happened within that time! The teacher merely just apologized but i find this matter really grave! What would you do?

This certainly shouldn't be taken lightly as it concern safety of the children. I would suggest bringing it up to the authority as well so that it could be highlighted. Regardless how busy the teacher is, a child should not be left unattended. More so during such timings where caregivers are coming to fetch their children. Every child should be accounted for and only released to their respective caregivers. Hope your niece (and your sister!) did not suffer any shock from this...