The first and most important thing a teacher should do if an autistic child is having a meltdown is to reassure them that everything will be okay and that they are safe. A meltdown is not a tantrum, rather a kid in crisis.
See the full interview with Eric Garcia: https://www.readingrockets.org/teaching/experts/eric-garcia
For more resources on ASD, visit Reading Rockets: https://www.readingrockets.org/reading-topics/autism-spectrum-disorder
See more Reading Rockets interviews with experts: https://www.readingrockets.org/teaching/experts