It's no schoolboy prank: A mysterious odor has shut down a fourth-grade classroom in southwestern Michigan.

The room at Clarke Elementary School in St. Joseph has been closed for about three weeks. Superintendent Ann Cardon tells The Herald-Palladium that the source of the odor still is unknown.

She says the district is waiting for air samples to be analyzed.

Officials thought it might be linked to a leaky roof. But a repair didn't eliminate the problem. Cardon says the bad smell seems to come and go.

A teacher says the odor was present last year but went away.