NAIROBI (Reuters) - A secondary school in rural Burundi sent home 230 students on Tuesday for defacing the portrait of President Pierre Nkurunziza in their textbooks, a regional head of education said.

Burundi has been mired in a year-long crisis in which more than 450 people have been killed since Nkurunziza pursued and won a third term. Opponents said his move violated the constitution and a deal that ended a civil war in 2005.

Kwizera Guillaume, the provincial director of education in Ruyigi province, 200 km (124 miles) east of the capital Bujumbura, said they had sent home a total of four classes who were sitting an exam.

"It is an option we took to push them to reveal the authors of the acts," he told Reuters by phone.

Earlier this month police clashed with students protesting at the arrest of other students for a similar offense of defacing the president's portrait in Muramvya, which is 50 km east of Bujumbura.

Another 300 students were sent home by their school in Bujumbura for defacing portraits of the president.

(Reporting by Duncan Miriri; Editing by Gareth Jones)