For Tamiflu to work on the flu virus you must take the Tamiflu within 2 days after noticing the symptoms of the flu.
After Taking Tamiflu you should no longer be contagious with the flu after 24 to 48 hours so it would be best to wait at least 48 hours after taking Tamiflu before returning to work or school.
After 48 hours you should no longer be contagious with the flu after you take Tamiflu as long as you take it within 2 day of coming down with the symptoms of the flu.