An end-of-term check at Dame Alice Owen School revealed layer upon layer of line applications on hockey and football pitches.
The school in Potters Bar, Hertfordshire, called in synthetic-turf firm Technical Surfaces to remark pitches.
"If left unchecked, health and safety can be jeopardised by seemingly innocuous slip and trip hazards causing accidents," said a spokesman for the firm.
"Alongside standard health-and-safety concerns of carpet rips and flooding lurks a hidden danger that might not immediately present itself.
"Numerous applications of football and hockey pitch markings over the years had caused a gradual accrual of paint build-up across the surfaces at the school."
Accidents could lead to facility closures, loss of revenue and personal injury lawsuit, he said.