This can first be attained at the beginning of Chapter 2. Swim towards the jetstream and turn right to enter it. Every now and again you will see small groups of pink fish which when you swim through them will turn to a white light (indicating that it has registered towards the achievement).
This jetstream doesn't contain 15 schools of fish but once you collect about 10 schools you can press Start and pick Main Menu. When you arrive back at the Main Menu press Continue to start at the beginning of the jetstream. Now you will need to simply collect about 5 more schools to unlock the achievement.