First off, I'm not going to comment on the private instruction high school students are or are not getting, except to say this: It's rare for a high school orchestra to have more than 20% of its members studying priately on their instruments. For the other 80%, the only technical advancement they'll get will come through the school orchestra. If your group has 100% private study going, good for you! You don't need to be reading this unless you want to ponder how the other 99% of us get along (not a bad idea, actually.)
There's a strong temptation to focus more exclusively on literature at the high school level, but this is a huge mistake. Concert literature simply cannot be used be used to build good technique, but focusing on direct technical instruction will give students what they need to handle performance literature. Skills will stick with them and carry over, eliminating that trip back to "square one" for every new piece of music.
My solution to this problem is "Scales, Arpeggios and Chorales", by Ian Edlund, published by String Instrument Specialists and available from your sheet music dealer. This material covers what I consider essential technique for high school orchestras and allows the teacher lots of room for creative applications. It presents unbroken two-octave scales (major and minor) through four sharps and four flats, with corresponding arpeggios, carefully fingered for each instrument. There is also a Bach-harmonized chorale in each key. I suggest pairing this material with "A Rhythm a Week", by Anne Witt, which does a great job of building rhythmic reading and understanding through exercises very well-written (idiomatic) for strings. Work with this material should occupy 20-25% of class time; however, with performance demands at this level being what they are, that 25% figure may not happen every day, or even every week. But over time it needs to average 25%.
That leaves 75% of the time for "real" music. The main focus at this level should be on masterwork-caliber pieces for both full and string orchestra, using works from all musical periods, cultures and styles. Bear in mind that we are educators, not entertainers, and everything we do needs to reflect that. Beware of pop tunes and movie music--cheese is still cheese. The criteria of involvement for all the instruments, appropriateness to instructional goals, and inherent musical value should still apply.
It's always great to play original works, but don't downplay the value of a well-done arrangement which might better suit your needs. And, if a concert includes five works, only one or two should really tax the group (but remember: everything has to sound good.) Two should be less demanding, needing only a moderate amount of rehearsal/teaching time. One piece should be undemanding but still contain teachable moments and be of interest to listeners. There's also a place for "rehearsal room music" which will never see the concert stage but still can provide valuable experiences for students. Teachers using these ideas as guidelines will give their students a broad range of musical experiences over course of a year.