Officials in the town of Jowhar say al-Shabab has ordered all schools to stop using bells to start or end classes because they sound like church bells.
Teachers and residents say they have to obey for fear of reprisals. Somali Islamist groups have killed opponents and carried out amputations, stoning, and lashings for alleged violations of Islamic law.
Al-Shabab and fellow militant group Hizbul Islam are trying to impose a strict version of Islamic law in parts of the country under their control.
Earlier this week, Hizbul Islam banned radio stations in Mogadishu from playing music. Most stations have complied with the ban.
The transitional Somali government, which controls only small parts of the capital, has not been able to stop the groups.