The demonstration started at 7:30 a.m. Wednesday at the intersection of Ventura and Coldwater Canyon.
Havard Westlake wants to build a three-story athletic field, parking structure and bridge across Coldwater Canyon in their Parking, Safety & Athletic Improvement Project.
Opponents argue that the proposed construction would complicate commutes into the Westside for at least three years.
There are also other safety and environmental concerns associated with the plan. Construction would permanently remove more than three acres of rare oak and walnut woodland.
Defending their plan, the school said the project would improve traffic flow by adding an extra through lane on Coldwater Canyon.
President of Harvard-Westlake, Rick Commons, recognized the members of the community against the project.
"Our democracy protects the right of our project's opponents to organize peaceful protests like the one this morning," Commons said. "This is a value we teach on our own campus."
Commons said the school's primary focus is not the morning protest, but continuing to conduct class.
This is the second protest against the development plan.