How can a pv2 be at blc before becoming a spc or even a pfc?

There is no rank requirement for BLC. While the priority of the seats go to promotable soldiers, there is no requirement to be promotable in order to attend ncoes.

There was a time when it was possible. Since the full implementation of the STEP program it is no longer possible as far as I know.

Waiver? There's a waiver for everything in the military, if the school had an open seat, and the Soldiers leadership/unit was trying to fill that seat, and if a waiver was possible, then boom, there you go, it happened.

Of course, this is basically unheard of, but anything can happen in the U.S. Army, right!