Anyone knows what's this? My son's teacher just told me he suddenly developed this rash all over. What's the cause of it and recommended remedies? Thanks!

It looks like hive and could be accompanied with itchiness at areas affected. It could be due to certain food or environmental allergies or insect bites. Maybe you can ask your child if he had eaten or touch something different. You can apply cold compresses or wet cloth to the areas affected. Stay in a cool room and wear loosely fitted clothes. It can stay from few hours or up till a day. It will still be good if you can bring him to see a doctor if the swelling doesn't go away just in case it is something else.