The elementary school in the central Illinois community of Ashmore was closed after half its 100 students failed to show up.
Community Unit School District No. 1 Superintendent Jim Littleford said Wednesday that in addition to 51 students, 5 of the school's staff also reported a flu-like illness.
Littleford said the Coles County Health Department suggested a thorough cleaning of the school Wednesday, which will allow it to reopen Thursday.
Littleford said officials are urging sick students to see physicians. Parents were urged to inform school officials of test results because it could help with an investigation into the cause of the illness.
He said there were no reports of the illness earlier in the week and none of the district's other schools had more than a few students out.