Here is a great set that I did with my advanced 11-14yr olds the other night. Challenged all the swimmers. I let them choose a name and they came up with a good one.
Ironman IM Set
Free Swimming Workouts, Sets, Ideas, and Dryland Exercises from Professional Coaches Around the World
The Swimming Wizard 
super set really, i tried it with our team 13,14 age group and it was so so hard for them and we used in the last part fins
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