The Japanese embassy in Beijing is on the alert for harassment against Japanese entities in China after reports of recent attacks.
Japanese diplomats believe the attacks are related to the arrest of the captain of a Chinese trawler in Japanese territorial waters near the disputed islands of Senkaku.
The Japanese School in Tianjin said someone drove to the school on Sunday evening and threw small iron balls at the building, breaking windows in the security guard's office.
Graffiti was found on the school's fences saying "Chinese people do not tolerate territorial violations."
The Japanese consulate in the southern province of Guangzhou said a man hurled a beer bottle against its wall on Thursday, one day after the captain's arrest. The man was caught by military police guards.
No one was injured in either Tianjin or Guangzhou.
The Japanese embassy has asked Chinese police to tighten security around Japanese entities.