Responsibility for Wednesday's attack was claimed by a Sunni group, Jundallah [God's soldiers], which Iran has said is linked to Osama bin Laden's al-Qaeda network.
Tehran has blamed Jundallah for past killings in the area bordering Pakistan, but it is not yet clear whether the same group was behind Friday's explosion.
An Iranian news agency said electricity was cut off after the explosion in the capital of Sistan-Baluchestan province.
"The bandits were shooting at people after the bomb went off at a school. Clashes are still going on between police and the armed bandits," the agency said.
Iran has accused Britain and the US of supporting ethnic minority rebels operating in sensitive border areas in an attempt to destabilise the country.
Iranian officials said five of those behind Wednesday's bombing, including the key suspect, were arrested by security forces.
One Iranian agency quoted an unnamed official as saying the detained men had received training from the US in order to create ethnic divisions in Iran.
The upsurge in unrest in Sistan-Baluchestan follows violence in Iran's oil-rich southwestern province of Khuzestan, which has a minority Arab population.