Instead of the traditional cross, activists encouraged people to write the number 43 in ash on their foreheads and to take to the streets.
Many people posted photos of their ash-inscribed foreheads. And the associated hashtag, #43nosonceniza ("43 are not ashes"), was trending for much of the day in Mexico.
The missing students, who attended a rural teachers college, were last seen in September in Iguala, a town in the state of Guerrero.
According to Mexican Attorney General Murillo Karam, a criminal group in the area mistook the students for members of a rival gang. They captured and killed the students and then threw their burned remains into a river, he said.
Former Iguala Mayor Jose Luis Abarca has been charged in the case and is awaiting trial, accused of being the mastermind behind the abductions and killings.
Parents don't buy official line
But the parents of the students refuse to believe the official line, even suggesting the Mexican military might've been behind the disappearance of the young men -- a claim denied by authorities.
The students were left-wing, anti-government activists, mostly in their late teens and early twenties. The remains of only one of them have been positively identified and parents are asking what happened to the other 42.
"The version from the Attorney General's office cannot be a "historic truth" because no one can ignore the witnesses, the lines of investigation and not listening to the victims," the activists said in their call to action for Ash Wednesday.
"We will send a convincing message through a pagan appropriation of the catholic rite of Ash Wednesday," they said.
In the Western Christian church, Ash Wednesday is the first day of Lent.
It was unclear whether the calls drew many people to gather in the streets, but they gained a lot of traction on social media.
The #43nosonceniza tag was mentioned around 34,000 times on Twitter over a 24-hour period, according to Topsy, a site that analyzes social media activity.