I am not a teacher, but I think it depends on the context.
So, you should sayonce
n. one time, single occurrence
conj. at the moment that, if ever
adv. one time; at one point in time; in the past
Once I heard that... (in the past)
Other uses of once would be
"I will say it only once"
"Once you start there's no turning back"
- For Teachers