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!

Looks like hives. Normally i will leave it alone inless it is very itchy. If it's itchy, will just apply some lotion. At the same time, will try to check on the food taken to rule out food allergies. Sometimes, it could also be dust allergies.