In High School Volleyball (FED rules), is it legal to use a one handed set and it goes over the net?

There is nothing illegal about a one-handed set going over the net. However, if the player is a back-row player and violates the back-row attack rule, that would be illegal.

Is there a specific situation or was a call made that prompted this question?

There was no situation. I was just wondering because sometimes I see the girls tipping the ball over the net when its real close the the net, but if they decided to set the ball over, but with only 1 hand. Its almost like a tip, but their hand is under the ball more. If not done cleanly, it woulf be a carry,I just wondered if setting with 1 hand would be a carry...

Thanks for the clarification. As with any other contact of the ball, setting with one hand would have to be judged as it happened. The act of setting with one hand is not illegal in itself, but the contact may be judged illegal.

One handed set
 

The basic criteria for the one handed set is the length of contact. It would be difficult to have a double hit with just one hand, but I have heard of officials who have called it...Allow the same amount of time as you would on a power tip, control block or "deep dish" set. See the case book for details about "prolonged contact.
Bob