Ijebu-OdeOver 100 Ogun pupils were reportedly hospitalised after inhaling gas.
It is the fourth school exposure since April, with over 300 children and teachers affected.
State officials link the emissions to underground gas along fault lines, after May readings peaked near 13,500 parts per million.
The state government says investigators have intensified inquiries, five months after it promised a multi-agency assessment.
How each outlet framed it
- Premium Times Nigeria
- names the affected schools and reports the state's 'under control' statement alongside the 300-plus cumulative pupil and teacher cases since April
Sources: Premium Times Nigeria