In reply to Trangia: I'm not an instructor myself, but I've helped with outdoor instructing groups, and once sadly got roped into helping belay and supervise a group of beginners at a local sandstone crag (luckily, one of the less classic crags, but that's no excuse).
It had rained massively in the days running up to the group's session, and the crag was in one place running with water, and was significantly wet in all of the cracks. Despite that, top ropes were set up, and the group was allowed to climb...
Sad to say, the attitude of the instructor seemed to be that the day couldn't be rearranged, as it was at too short notice, and the group probably wouldn't come back if it had to be rearranged anyway.