Officials of the new Ivy Academy charter school have been given until the evening of Aug. 16 to have a certificate of occupancy secured.
Rick Smith, associate superintendent of the Hamilton County Schools, said that is a "drop dead" date.
He said officials of the environmental-focused school believe they can be ready by then at the location on Dayton Pike in Soddy Daisy.
Mr. Smith said the group has given up on using a temporary site such as the Convention Center.
Ivy Academy plans to open with 54 ninth grade students.
Janice Boydston was the only board member to vote against the extension.
She said she has concerns in several areas, including meeting requirements on food service, transportation and special education.
Ms. Boydston said she saw video of the school site on TV on Monday night. She said, "What I saw was cinder blocks on both ends of portables and dirt and mud all around.
The first local charter school, the Chattanooga Girls Leadership Academy, has already opened.
Wednesday is the first day of school for other county school students.
Mr. Smith said officials of Ivy Academy are very familiar with the students who plan to attend, so missing registration day is not critical.