Back to School is a full featured Left 4 Dead and Left 4 Dead 2 campaign. The storyline is not interfere with none of this games and offer completly new setting of Borden city, struggling to survive in an epidemic apocalypse alongside with original 4 heroes. They are getting a chance to save themselves, since USA marines are coming in town and not falling back this time. But during the battle apogee something goes terribly wrong and local school - the most safe place in city - turns out to be a death trap.
Has the quality of an official Valve L4D campaign, and I like it more than L4D 2's as a matter of fact.