Diving at quarry with training platforms and instructors strealing platforms from divers is rude. I've had it happen multiple times, most recently Saturday......He and I went down to a platform and were working on buoyancy and trim (he had a new BC among other things). Viz was very bad (10ft or less) so we didn't go see some of the attractions as originally planned (I didn't trust my so-so nav skills with a diver with few dives). We were swimming around the platform when along comes an OW class and steals it from us.
If you're talking about Haigh quarry, yup, I know the feeling. Had it happen to me too. I don't get upset because I know a class has to use the platform for training so I just move to another platform. There are a few of them at the north end all fairly close together.
P.S. If you're going to Haigh and need a buddy or if you need a buddy in the future PM your email address and I'll respond back with mine. Maybe we can get together sometime.