السلام عليكم ورحمة الله وبركاته أحسن الله إليكم Sometimes there is only one brother and one sister present in the school musallah, with a barrier separating the brothers’ and sisters’ sides. Is this considered khalwah /impermissible mixing?

Ustadh AbdulAziz al-Haqqan
No, it is not khalwah, since they are separated.