Who knows? I don't personally know the truth. I would like to think that he WAS at the school, because that makes him less of a liar, and it pains me to think of people in positions of power as liars. We have more than enough already.
In a little while it won't matter -- the official narrative will rule, any statements to the contrary will have been purged, all discussion will cease.
If his account of being in the school building and hiding with his classmates (having been warned by a janitor) is original, and his account of returning to the school at 6 p.m. is also original, then we have our answer
If nobody on the conservative side cares to investigate, but just continues to state categorically that he wasn't at the school at the time of the shooting, then we are no better than the liars and obfuscators on the other side.
We should be seeking the truth, even if it means we can't attack Hogg on this point. There are plenty of targets of opportunity.