The Sunni Muslim-led Gulf kingdom, which hosts the U.S. Fifth Fleet, is keen to show progress on reforms it pledged after international criticism of its crackdown on demonstrations led by members of its Shi'ite majority. At least 1,000 people were detained during the unrest and several died under torture, by Bahrain's own admission.

Opposition parties say the government is merely trying to impress Western allies which had pushed it to allow an independent investigation of its handling of the protests and abuses committed during a period of martial law.

HoweverTeacher's union's chairman, Mehdi Abu Deeb, and his deputy, Jalila al-Salman, remain on trial, said Mohsen al-Alawi, a lawyer for two of the acquitted teachers.Abu Deeb and Salman are accused of disrupting schooling, broadcasting false news and threatening national security by encouraging protest marches and sit-ins.

Rights groups and foreign governments have condemned Bahrain for other crackdown-related prosecutions, including a trial now under way of 20 medical staff who treated wounded protesters.

A year ago the authorities quelled demonstrations seeking democratic reforms, curbs on the ruling family's power and an end to sectarian discrimination. A few Shi'ite groups had also called for the monarchy to be replaced with a republic.

Bahrain's rulers, who called in troops from fellow Sunni monarchies Saudi Arabia and the United Arab Emirates as they crushed protests, say Shi'ite giant Iran is fomenting unrest among its co-religionists in the country. Iran denies this.