A school science block was evacuated as a precaution after chemical fears.
Students were cleared from the second floor science block at Royds Hall Community School in Paddock on Friday morning.
A two-litre bottle of ammonia, stored in a sealed cupboard, had overheated and began emitting a strong odour and firefighters were called.
Watch commander Chris Bell, of Huddersfield Fire Station, said staff at the school wanted fire crews there as a precaution while the bottle was opened and the ammonia put into another container.
Three fire engines attended and firefighters had breathing apparatus ready if needed.
The alert, which started just before 10am, was over in about 20 minutes and no-one was injured.
Mr Bell said crews were called as a precaution in case of any risk.
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