Reference article
Stage (Level 1)
Stage level 1 lets one teacher train 2 learners together.
Building it
Upgrade to 
2 to make Stage available, then select it from the building menu.
Teaching your flock
Choose a skilled resident as the teacher and seat up to 2 other ducks as learners. The teacher has a separate place. Learners improve racing skills where the teacher is ahead of them; 
Energy still comes from feeding. Upgrading adds more learner seats.