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...
Totally not acceptable! Your sister definitely needs to bring this up with the school - not for an apology - but to make sure the school revises it's safety practices and no other kid has a similar incident.
Definitely report to higher authorities, an apology is not enough. This time nothing bad happened but in the future who knows? What's the name of the school? You can report to MOE or MSF if I'm not wrong.
Pls share the name of the school! So we all know which school to avoid. How scary. I hope your sister is doing okay. I would be horrified if this happened to my child.
Woah this is really crazy. Did you speak to the pricipal of the school? This is very serious. Please don't let it go - it could have had very grave consequences!!!!
Definitely bring it up to a higher authority. This is no laughing matter