Vandale have targeted Warrawong Public School, smashing windows and trashing offices and classrooms.
Inspector Andrew Koutsoufis, from Lake Illawarra police, said the offenders entered the school's buildings on Friday night by smashing ground-level windows and went into a number of classrooms before ransacking the premises.
Police estimated the damage to be worth about $1800.
A Department of Education spokeswoman confirmed a break-in occurred at the school over the weekend, with windows and locks damaged and some vandalism to offices.
Insp Koutsoufis said no property had been listed as stolen.
According to information from the Department of Education, security fencing has been in place around the school since 2003.
The break-in follows several robberies at Illawarra schools this year, where thieves stole copper pipe from buildings.
Lake Heights Public School and Coledale Public School both had thefts, while an attempt was made at Scarborough Public School.