When you sleep on your stomach in class, you suddenly shake, mostly when you sleep, the center of the brain for the muscles of the limbs of the dominant function decreases, resulting in a physiological phenomenon.
Classes lying down to sleep will suddenly shake, if this situation is only occasional, wake up limb activity is completely normal, it is just a normal physiological phenomenon.
The human body in sleep, or from awake into the sleep state, the function of the nervous system will be switched, the cerebral cortex of the nerve center for the limbs of the muscle control decreased, the muscles may produce some unconscious movement, such as a sudden shake.
There are a few diseases that may also cause shaking during sleep, such as epileptic seizures, calcium deficiency, and so on. But people with epilepsy often have no memory of the seizure when they wake up. Calcium deficiency, on the other hand, tends to cause twitching of the arms and legs, or even pain because of the twitching, and usually not just a simple shaking.
If this persists, you can go to the hospital to get further clarification of the cause.