Simple. I would assume, based on the number of Turok-han, that it would nearly be suicide going down to the hellmouth, so instead of blocking basement exits, I would block the entrance to basement room over the seal, and post some of the Potentials and 'civilians' outside the door, just in case. Kennedy and another potential would be with Willow in the office. I would then have the rest of the potentials, about 2/3 of them, + Buffy, Faith and Spike, go down into the hellmouth, while 1/12 stay in the seal room as back up. I would station 3 slayers to defend spike while the others just kill turok han. Willow would set the spell off before they went down into the hellmouth, but not to soon so the First would realise and prepare. Buffy would call her on a radio/mobile phone.
When Spike's amulet starts up, They could get the slayer's outside to unblock the exit so they can escape. Any Bringer's that come along, will be easily slayed. And the Gang can escape in the preprepared schhol bus.
There you go, much better thought out. But considering how well the battle went on the real thing, i would have felt it unnecessary. Of course, in real life, with continuity working, Buffy's plan would have sucked.