How to Determine if Your Child Should Stay Home:
Fever
100.4 degrees or higher
Keep home until the fever is gone for 24 hours without medicine
If fever is accompanied by headache and stiff neck immediately seek medical attention
Mild Cough/Runny Nose
If there's no fever and the child feels fairly well, school is fine.
Bad Cough/Cold Symptoms
Children with bad coughs need to stay home and possibly see a doctor. It could be a severe cold or possibly bronchitis, flu, or pneumonia. But when the cough improves and the child is feeling better, then it's back to school. Don't wait for the cough to disappear entirely -- that could take a week or longer!
Diarrhea or Vomiting
Keep your child home until the illness is over, and for 24 hours after the last episode (without medicine). Though diarrhea and vomiting are not usually associated with flu, they may sometimes occur with swine flu.
Sore Throat
A minor sore throat is usually not a problem, but a severe sore throat could be strep throat even if there is no fever. Other symptoms of strep throat in children are headache and stomach upset. Keep your child home from school and contact a doctor. Your child needs a special test to determine if it is strep throat. He or she can return to school 24 hours after antibiotic treatment begins.
Earache
The child needs to see a doctor.
Pink Eye (Conjunctivitis)
Keep the child home until a doctor has given the OK to return to school. Pink eye is highly contagious and most cases are caused by a virus, which will not respond to an antibiotic. Bacterial conjunctivitis will require an antibiotic; your doctor will be able to determine if this is the case.
Rash
Children with a skin rash should see a doctor, as this could be one of several infectious diseases. One possibility is impetigo, a bacterial skin infection that is very contagious and requires antibiotic treatment
Lice/Nits and Scabies
Follow treatment guidelines from your Health Care Provider, school official and/or over the counter treatment protocol
Once your child has followed the treatment guidelines and the nits (eggs) are pulled, bring him/her to school to have the school nurse or designee recheck for re-admittance to school. The child must be live lice and nit-free to return to school.
Source: http://www.webmd.com/cold-and-flu/features/your-child-too-sick-for-school