At least 21 children and five adults died at a primary school which collapsed in Mexico City during Tuesday’s quake, the government says.
The shallow, 7.1 magnitude quake killed at least 230 people in total and caused major damage across states in the centre of the country.
Desperate searches for survivors under the rubble are continuing.
Hopes rose at the school when a girl was located alive and a delicate operation to rescue her is under way.
Dozens of buildings collapsed across the country, including several churches where worshippers were killed.
Families with young children could be seen sleeping outside their homes in the street in the capital.
President Enrique Peña Nieto has declared three days of mourning.
The tremor struck shortly after many people had taken part in an earthquake drill, exactly 32 years after another quake killed thousands in the capital.
Mexico is prone to earthquakes and earlier this month an 8.1 magnitude tremor in the south left at least 90 people dead.
Though it struck a similar region, Tuesday’s earthquake does not appear to be connected with the quake on 7 September, which was at least 30 times more energetic, the BBC’s Jonathan Amos writes.
Enrique Rébsamen primary school, in Mexico City’s southern Coapa district, collapsed.
Confirming the death toll, Education Minister Aurelio Nuño said 11 people had been rescued while two children and an adult were missing.
Local media had reported a higher death toll earlier.
More than 500 members of the army and navy, along with 200 police officers and volunteers, have been working at the site, Mexican newspaper Milenio says.
Details of the girl located on Wednesday have not been given. Rescuers detected her after she moved her hand and a hose was lowered to supply her with water.
Civil Protection volunteer Enrique Gardia told the assembled crowd that a thermal scanner had detected several survivors trapped between slabs of concrete.
“They are alive! Alive!” he shouted.
“Someone hit a wall several times in one place, and in another there was a response to light signals with a lamp,” he added.
One mother, standing nearby waiting for news of her seven-year-old daughter, told reporters: “No-one can possibly imagine the pain I’m in right now.”
At least 209 schools were affected by the quake, 15 of which have suffered severe damage. (Courtesy BBC)