A person infected with chickenpox is advised to stay home and away from infants and small children or other non-immune people for 21 days after exposure. This is the incubation period before illness occurs. If the person does not become ill in 21 days, he/she may return to usual activities around others. You can speak to the college and get a letter from a dermatologist confirming the chicken pox and show it to the college, in some colleges you're allowed to sit in isolation and write it, if the exam is important then such methods can be tried.