Activists said a school employee was killed and several students hurt, four seriously, when a shell exploded near a school bus.
Security forces in several other parts of the country fired on crowds holding overnight demonstrations, causing casualties, activists said.
The new attack using military forces pointed to Assad's determination to crush the two-month-old revolt, despite US and European sanctions, including an EU assets freeze and a visa ban on Assad and nine members of his regime.
The uprising, which began in mid-March, is posing the most serious challenge to his family's 40-year rule. What began as a disparate movement demanding reforms has grown into a resilient uprising seeking Assad's ouster. Human rights groups say more than 1,000 people have been killed in the crackdown.
The military attacks targeted the towns of Rastan, Talbiseh and Teir Maaleh in the central province of Homs.
Authorities had sealed off and isolated the towns by closing roads and cutting phone service, the activists said.
"The towns are under siege," one of the activists said. The activists spoke on condition of anonymity, fearing government reprisals.