The first Merfolk spell you
castTo cast a spell is to take it from the zone it’s in (usually the hand), put it on the stack, and pay its costs, so that it will eventually resolve and have its effect. A player may cast a spell if they have priority. See rule 601, “Casting Spells.”
To cast a card is to cast it as a spell. (CR 701.5) each turn has
convokeConvoke is a static ability that functions while the spell with convoke is on the stack. “Convoke” means “For each colored mana in this spell’s total cost, you may tap an untapped creature of that color you control rather than pay that mana. For each generic mana in this spell’s total cost, you may tap an untapped creature you control rather than pay that mana.”
The convoke ability isn’t an additional or alternative cost and applies only after the total cost of the spell with convoke is determined.
A creature tapped to pay for mana in a spell’s total cost this way is said to have “convoked” that spell.
Multiple instances of convoke on the same spell are redundant. (CR 702.51).
At the beginning of your end step, if you control two or more tapped creatures, conjure a card named Stonybrook Schoolmaster onto the battlefield tapped.