When you learn new scales have everyone finger notes 4 and 7 and look around the room to see who's fingering it incorrectly. These are typically the most frequently missed notes in scales.
It sounds as though you need to review key signatures and accidentals with the band. Some kids, especially younger kids, don't always grasp or retain this easily. Be sure that they understand and remember that an accidental is not only good for the remainder of a measure, it applies to octaves, as well. Sometimes kids will play an E natural for a note and then play it as E flat in the same measure simply because the E is higher or lower than the one with the natural sign.