Human Rights Watch (HRW) has urged Iranian authorities to immediately free seven jailed teachers and to end the harassment of dozens of others.
The New York based group issued the call on October 5, which is also World Teachers Day.
In a statement, HRW said Iranian authorities had detained at least 39 teachers since 2009 on various national-security-related charges.
It said all 39 were "targeted for exercising their rights under international law of freedom of speech and association, and peaceful assembly."
Fifteen of them have been sentenced to prison, mostly in connection with trade union activities.
Seven remain in jail. Among those released, HRW said, some still have pending charges and could be summoned to court at any time.