Tap on a fish to be the leader of your school, it is best to pick a larger fish as smaller fish will follow bigger fish. Drag your lead fish past others of the same species to get them to join in swimming. Sometimes you may need to go back if some don't join in though! Once you have your school, keep swimming them around as a group with your leader to fill up and complete the mission bar or if it is for a picture, release the leader and snap a pic!