The army's size makes it a military problem and military problems have military solutions. Surrounding the school with massed machine guns and armour or simply killing the Turok-Han as they emerged from the Hellmouth, which they could only do a few at a time, would have worked. If only Buffy knew someone in the US military she could call...
flame throwers maybe. I wouldn't trust machine guns to work easily. Would the military get there in time? How many men would Riley be able to gather?
I think Kennedy had potential. No pun intended! But she needed good leadership to direct that potential.
By this point, Xander has more leadership experience than any of the Scoobies and has spent over six years fighting creatures that are stronger and tougher than him and not dying. Not even Buffy can claim the last. He has more fighting experience than many military veterans. Anya has a couple of years of similar experience.
Xander had never trained anyone to fight. Nothing suggests he really knows how to fight. Same it true for Anya.