The 150-year-old primary school, which also houses Athy VTOS, has been extensively damaged.
The blaze broke out at around 4am this morning.
Gardai are appealing for witnesses, particularly anyone who might have been on the Dublin or Kildare Road between 12 midnight and 4 or 4.30 this morning.
A picture of the Model School in Athy taken during this winter's snowfall
The scene is being preserved for examination and gardai have asked that passers-by do not attempt to visit the site.
The 100-plus pupils at the Model School are now without accommodation, but other schools in the area are going to help, in time for class next week.
