A fire at a rural Waikato school is being treated as suspicious, the second arson at a school in the region in a week.

However, the damage was able to be limited to a classroom at Glen Massey School, five minutes west of Ngaruawahia, after the actions of quick-thinking residents.

Firefighters were called to the school just after 7 this morning.

School principal Tim Howard said damage was confined to the 100-year-old classroom which had three of its small windows smashed yesterday, the sixth vandalism at the school since March.

Mr Howard believes the classroom will be able to be saved but the school will have to close for the next two days as they find an alternative room.