I definitely remember that this story was a written work of fantasy, meant for children, and was fairly short, so probably a short story, not a novel. I read it approximately 5-10 years ago.
The plot involved children, a teacher, and a magical garden. The teacher discovered that people could turn into animals in the garden, so she had the practice of her students doing so in that location.
What sticks out in my mind are the things the teacher made her students do so that they would be safe. For instance...
Her students had to learn how to keep track of time and set mental timers exactly, even without a clock.
The transformed people would have most of the animal's mind, but with enough human parts to be able to remember and process their experience after. Retaining too much humanity while in animal form would be dangerous.
- The teacher also made sure that students could not turn into the first choice animal they most wanted to be, because they ran the risk of being unable to return to being human.
I remember that the teacher had to specially deal with two children because one turned into an animal that was a predator of the other. Also, one girl did transform into the bird she most wanted to, and the others had to convince her to return to human form.
I've been trying to find this story for a while, but I couldn't find it by searching for the plot on Google.
Edit: I believe the name of the girl who became a bird had "Mary" in it, although I am not 100% sure.