Well, if it's like most trainers with a square fuselage, it's a lot easier to straighten than a wardbird or something with a bit more complicated fuselage.

If you lay it on the table and press down, will it remove the twist in the tail? If so, simply weight it down with some sandbags (1 gallon ziploc full of dirt work for this) or some books... and glue the seeting in place. Usually, the sheeting adds enough rigidity to remove all or most of a warp in a situation like this. Just be sure it's straight before you go gluing the sheeting on - it's going to be much harder to straighten after it's sheeted.