The children gave speeches and carried colored papers with messages urging people to say "no" to Valentine's Day, the Jakarta Post reported.
"We took the students to join the rally in a bid to remind them not to fall into Valentine's Day moments," a teacher identified as Setiyatno said.
The teacher said the romantic holiday promoted activities that could damage the morality of Islamic youths.
"What we see in reality is that youths increasingly have no limits when celebrating Valentine's Day," he said.
A fifth-grader identified as Khodir told those attending the rally in the city's stadium Valentine's Day must not be celebrated by Muslims because Islamic teachings direct unmarried couples against touching each other.
Pekanbaru Mayor Ayat Cahyadi said Wednesday he had told school officials in his area to ban an Valentine's Day observances.
"The young generation must be protected so that its does not fall into promiscuity and vice," Ayat said.