This depends on the progress and ability of your pupil. Most people teach forward somersault first as its easier for pupils to create the take off phase of the skills whereas the take off for a backwards somersault takes a lot more effort and skill. As long as all the pre-req skills have been mastered there is no reason why you cannot teach Backwards Somersault before Forward Somersault.
Personally I teach forwards first as there are plenty of forward rotating skills to prepare and practice as pre-req skills so its a natural progression to move onto Forwards Somersaults. I always consolidate each new skill, then link it to other skills and gradually introduce it into routines. Once the Forward Somersault can be performed in a 10 bounce routine with no effort or issues, I then give them some space to grow before looking at moving on to teaching the Backwards Somersault.