One is a 12-year-old girl who is believed to be sheltering under a table, but rescuers say they do not know how to reach her.
At least 21 children and five adults died as the school collapsed.
It was one of dozens of buildings toppled by the quake. At least 230 people are known to have died.
President Enrique Peña Nieto has declared three days of mourning for the victims.
As rescue operations continued for a second day, attention was focused on the Enrique Rébsamen primary school, in Mexico City’s southern Coapa district, where the body of a female teacher was found overnight.
With anxious parents gathered outside awaiting news of missing children, civil protection volunteer Enrique Gardia announced that a thermal scanner had detected 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.
Rescuers at times were demanding “total silence” from bystanders around them in order to listen out for any calls from survivors.
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.”