“The Twitter feed for Lucas Ames’ class in American history has shown some lively exchanges of ideas and opinions among students at the Flint Hill School. One day this month, 11th graders at the private school in Oakton, Va., shared articles on the separation of church and state, pondered the persistence of racism, and commented on tobacco regulation in Virginia now and during the Colonial period—all in the required Twitter format of 140 or fewer characters.”