I've been an orchestra teacher for a long time, and might be able to help a little. Have your students been trained to use finger patterns? All the string inst. follow the same fingering system regarding 1/2 & whole steps, they just use different fingers for different notes. Your students MAY respond well to instruction telling them to use a certain pattern in their left hand. Eg Open pattern on the A string with a low first finger would give them Open A, Bb, C,D & E..... each finger has a whole step between them. The Essential Elements for Strings books are a good starting place, the back of the 2nd book is loaded with finger pattern work. I'm sure there are others, and maybe your supervising teacher has an instrument and copies of books you could borrow. Even though you don't have time to learn to play the instrument, you could familiarize yourself with the finger patterns. Mandolin has has the exact same strings and notes as violin, with the advantage of being able to see the frets, thus the patterns, maybe that is another way you could get more familiar with them.
Another idea may be to have your section leaders address the problems. You can define them, then ask the section leader to demonstrate the difference between correct and incorrect. If the problem is remembering to play the Bb they can highlight the note. Flats are a notorious problem for class orchestras at any level. Remember, even if the students play better than you they don't have the discriminating ear that you have.
I hope this helps a little.