Amersham School is rated Good by Ofsted, I believe, and I would consider it a good school from personal experience.
The HT has really tried to clamp down on disciplinary issues of the past and we certainly didn't see any problems. DD has a serious ongoing medical condition which was, for the most part, dealt with both efficiently and sympathetically. She was certainly happy there and has a nice group of supportive friends.
As regards your facilities question:
Labs – yes, excellent. DD did triple science for GCSE, chemistry and biology for A level and has just started a biomedical science degree. She loves lab work and I'm sure we would have known if she thought facilities/resources were lacking
IT – sorry, can't help with that one, not DD's thing
Sport – yes, they have a very impressive new sports hall (which bears comparison with the one at DCGS), there are opportunities to try out a wide range of sports and activities. Plenty of school teams in Y7-11 in a variety of sports if your DS is into competitive sport
If I had younger children, I wouldn't hesitate to send them to Amersham School. HTH