We didn't like Dartford for our DS last year, either. And neither did he. We found the PE dept arrogant about the compulsory rugby. When we asked what they thought about DS having to give up a sport he plays at county level on Saturdays to accommodate their rugby training, the teacher just shrugged. Our way or the highway kind of thing. Which I am afraid really put my hackles up. The main reason we didn't choose it however was because we just couldn't see our DS fitting in there.
The other two schools you mention have a totally different attitude and seem to celebrate the individual more. Feel free to PM me for more information.
In my opinion, a school cannot be judged by a teacher or even the PE department. Dartford Grammar offers much more than that and their attitude, in all aspects, has been faultless towards my son and that of his friends. I meet with other mothers and have never heard a complaint about the school. Some of the boys are in football teams as well as them being in the A rugby team. They somehow manage; love going on rugby tours, etc.
If you have a choice of other schools, however and are not very sure, then think about the 90 or so OOC boys who will be on the waiting list. They will LOVE to go there and will benefit from people choosing elsewhere. I believe both Judd and Skinners are great schools. We didn't choose them, although our son had the score for them, because they were too far for us.