I hear of situations such as this quite often - A good all around player can help the team in so many positions, that they get 'plugged in' where needed, and switched so often, that it is hard to find a comfort zone.
Part of the solution will naturally come this next year. As she matures into a sophomore, and her team mates physically mature one more year, it may be easier for her to play in one position. For instance, if she does not grow much, and a couple of team mates sprout up, then she probably won't be playing middle blocker and will be concerned with the outside hitting position.
With school volleyball, VolleyFamilies are at the mercy of the coaches. Other than our taxes and an equipment fee, players don't pay to play school sports. If the coach wants the 6'5" player to just be a Libero, well.......you gotta take one for the 'team'. It is easy for me to say, but with school volleyball, you just gotta go with the flow, no matter how much hair you have ripped out of your head trying to stay mellow. As she develops in club, you will find that school volleyball may become nothing more than a good off season of ball touches for club volleyball.
Speaking of club volleyball, you pay to play so you have the ability to dictate her position. Again, physical maturity of her and her team mates will assist in getting her into a consistent position. I would expect that she will be in the OH slot because 5'4-8" MB's are not the norm for 16's club volleyball; this would be the OH position.
Another suggestion is to have her practice her spandex off to become a great OH. The pure OH is golden at all levels; this is the player who can pass, defend, hit, block and serve so consistently in the OH position that coaches must have them in that position. I know there are any number of knucklehead coaches out there, but it would take an extremely dense coach to not have a pure OH on the court at all times.
As she is all around good player, the coach has a valid argument for bouncing her into any gaps. Unless she makes an unreal growth spurt this summer, she will have the height of a traditional 16's OH, and that is the position she needs to target at becoming even better at - Since you say she is already a good back row player, then keep that passing skills elite, and tell her to focus upon her attacking ability on the left side.
Coach Matt Sonnichsen