It is important to realize that there is fundamental difference between the two types of educational environments highlighted in the title. No doubt about exam time in high school is an absolute nightmare and very stressful time for the students concerned. Part of your job as a professional educator is to get them through the whole experience, ideally in a position where every student has met their potential and can have the choices they want, concerning their immediate future. For preschool children whether you are from Danville, PA or downtown London it is more about congratulating and encouraging your child when he or she comes home with that days classroom creation.
Preschool children learn through play.
If high school students are allowed to deal with exam stress by having free time between study periods, then preschool learn how to get into that position by learning through play and interaction. It seems completely unrealistic to place a 3 or 4 your old in an educational setting where they are expected to not play in the classroom. In most high school classrooms play is not what is expected, attention to exposition and the best possible result is the desired outcome. Even in high school settings children will get bored without variety, a truism that is even more applicable to the preschool environment. Very young children need to learn about the world around them by playing with colors, by making hats, by dressing up, by playing in a wet and dry sand pit as well as growing their own plants.
Play sets the scene
Preschool is the environment where children learn through experience about self-expression and why the top heavy block tower they built fell over. At this stage it is not too important that they learn the why of something, just that it did and depending on the activity, whether or not to do it again. A child gets stung by an insect once and instinctively knows that they keep away the next time. In the preschool classroom they may learn that the insect was wasp and that to stop it hurting vinegar is necessary. They can learn about the relative concentration of hydrogen and hydroxide ions and why this cause neutralization in high school. In preschool it is the knowledge gained through experience which is important, the why comes later.