I beilieve it could be valuable in some classes because classes like art and gym are classes that kids have fun in. Classes like english and science are a little different to calculate because they don't usually do anything fun. Some kids like those classes but other don't. A lot of people in todays world walk around with what is called "resting b*tch face" where there is no expression and they do not want to take the time to look a certain way. This means that some people could be happy but not look it and visa versa. There is not a good way to get around that situation. Therefore the results would not be accurate. One good way to test if the students like their class is by giving them a survey and let them tell you them selves. I believe that this is a bad idea to test out on students.