Fish are amazing animals. They have very unique characteristics, compared to dissecting a rat for example. This activity/prac is quite long and is aimed at senior students (usually ran with Year 10 students). I have split it up into two parts: A lesson for an external 'dissection' and another lesson for an internal dissection. The lesson length at my school is 75 minutes and this is just enough time to complete the guided worksheets. It is best to plan to have the lessons as close together as possible as the fish need to be refrigerated in between, yet still smell pretty bad! I would recommend working outside for this activity as the smell, especially after the second lesson (internal dissection) is very strong.
You can choose how many fish you would like to have dissected in your room. All students (within pairs) could dissect the same fish, which makes it easy to point common features out to all students, or you may choose to have every second pair dissecting a different fish. This way students can easily compare and contrast their specimens towards the end of each lesson. I usually use Silver Trevally, as its quite large and inexpensive (purchased through a local fish monger), but have used smaller and more expensive fish (snapper) before to compare it to.
The worksheets guide the students though looking at the external features of the fish, which allow it to successfully survive. Features such as body, fin and mouth shape. The internal features are mostly centered on the digestive system (as its easiest to access).