"Students and staff may wear black and orange all day," but no costumes, the St. Therese of Lisieux Catholic Elementary School said in a letter.
Children not wearing black and orange were expected to wear the school uniform, the letter said.
The school said it would emphasize the Christian origin of Halloween by beginning the day with "a Liturgy of the Word."
Halloween is thought to have been influenced by the Christian All Saints' Day -- also known as Hallowmas, All Hallows or Hallowtide -- and All Souls' Day, Nov. 1 and 2 respectively.
"Halloween" as a word dates from the 16th century Scottish variant of the fuller All Hallows Even, or evening, the night before All Hallows Day.
Parent Lynda Fraser told CTV News the school's decision to "cancel" Halloween was a disservice to students.
"Halloween is an event all children look forward to every year," she wrote in an e-mail. "While they can still have their fun and celebration at home and in the evening, many children look forward to the school day celebration where they get to wear their costumes for their friends and have a class party."
Children tend to lose focus and get worse grades when they don't enjoy school, she said.