Masud Shafie, who represents teachers' union leader Rasul Bodaghi, told Radio Farda on May 31 that his client informed him he had been assaulted late last week in prison by three Revolutionary Guards.
The lawyer said there is no evidence to justify Bodaghi's continuing imprisonment and that his client was simply trying to improve Iran's educational system.
Bodaghi, a member of the board of directors of the Iranian Teachers Trade Association. is being held at Rajaee Shahr prison in Karaj, west of Tehran.
Bodaghi has been imprisoned since September 2009. His lawyer says the charges against the teacher include "holding assemblies in order to act against the national security" and "propaganda activities against the Iranian regime."
In the year since Iran's disputed presidential election, an increasing number of teacher activists have been dismissed, forced to retire, detained, or even sentenced to death.
Kurdish teacher Farzad Kamangar was among five people hanged in Tehran's Evin prison last month for alleged links to terrorist groups.