The job of children is to be educated so they can be productive adults. Although academics is a big deal, kids should be allowed to be kids as well. That means having fun, playing and spending time with their peers. Too much emphasis on academics can lead to demotivation of students over the long term and can cause burnout by the time high school graduation comes along.
Extra pressure of academics definitely leads to the demotivation of students. Some pressure is fine, but excessive pressure is extremely stressful and does more harm than good to students. Some people can handle a lot of pressure, but even those who can handle it will buckle under the pressure eventually.