To me, "fictional" is used when referring to a written work: "He later published a fictional account of his trip down the Amazon."
"Fictitious" would be the word to choose if you are talking about something other than a book: "You mean, you believe those stories she tells about her first marriage? Completely fictitious, my dear. Her ex-husband is my husband's second cousin, and we know all about it."
You could use "fictional" in the second example as well, but I would not use "fictitious" in the first.
[not a teacher]