Brian Watkins, 52, was demonstrating last April how to make a screwdriver handle using molten aluminium when there was an explosion, magistrates at Newport, Gwent, were told.
The design and technology teacher at Duffryn High School in Newport suffered injuries to an eye and an arm, and four sixth-form boys were splashed with molten metal. None of the pupils was seriously hurt.
Watkins, who denied the charge, was convicted of breaching the Health and Safety at Work Act by failing to take reasonable care of himself and others. No one in the classroom was wearing protective clothing.
The teacher has since retired due to ill-health.