Depends on the swim school.
The baby lessons at our swim school are about building their confidence. Songs and paddling (while being held) towards a toy thrown just in front of them. Or jumping from the side into the parents arms etc. Our swim school teaches to blow bubbles in the water not hold their breath. You have to go in with them.
But at just over 2.5yrs my twins were able to go to the lessons with the teacher on their own which is more about learning how to swim.
You just need a swimming costume and a towel and swimming nappy if she is still in nappies. Goggles if you want but not compulsory. Our swim school has swimming caps as compulsory but they give them out when you pay for the lessons.