Students say on break to convey that they are not attending school for a period of time:
I am currently on Christmas break.
When talking about the future and what someone is going to do on a school break, we often use for:
I am going to Florida for Christmas break. What about you, David?
To talk about a break from school in the past, we often use "over":
So, James, what did you do over Christmas break?
In addition, during can be used for the last two cases:
I am going to Florida during Christmas break. What about you, David?
So, James, what did you do during Christmas break?
Although it is not a preposition in this case, note that you can use off in a number of cases:
I have this week off for Christmas break. (Talking about the present)
I have next week off for Christmas break. (Talking about the future)
I had last week off for Christmas break. (Talking about the past)