A parent of student of the Rajvinit School spotted the object at 7:20 am and alerted police.
The school is having the last final exam day Wednesday.
Police and military bomb disposal officers were rushed to inspect the object, which turned out to be a hardened tarmac tied with electric wire.
The department is located on Uthong Nok Road and is also next to the Rajabhat Institutes Suan Sunantha and Suan Dusit.
The Nation