Help Don't Hinder
How you conduct can either help the students play more accurately or make it more difficult for them. Here are just a few examples of when and how you can help the students do their best:
Abrupt stops - An often-used effect in music is the abrupt stop. Yet, especially in junior high, there will be any number of students who keep playing. An effective way to solve the problem is to stop moving when the music is supposed to stop. Most of the time that will be at the ictus of a beat. Give the beat and stay there without any rebound. You'll be surprised how well it works.
Dynamic changes - How you conduct different dynamics is another way to help or hinder the performance. Your conducting pattern should be smaller when conducting soft passages and larger when conducting loud passages. If your conducting pattern is always the same size, it will be more difficult for the students to make the changes.
Long notes - If everyone is holding a long note, there is no need for you to beat every beat in the measure. In fact, by doing so those students who struggle to count and play at the same time will think it's time to move on. Just conduct the beat the long note starts on and then slowly raise your hands and resume conducting when it's time to play the next note. By raising your hands during the long note the students are less likely to stop playing and it also helps them support the tone to the end of the note.
Critical entrances - The beginning of a solo, a climatic cymbal crash, and that lone cowbell in David Holsinger's
Havendance are examples of times when you can be a big help. Start by making eye contact and pointing at the performer with your left hand well in advance of the entrance. Make the cue very deliberate and obvious so there is no doubt when it's time to play. If you don't help with the critical entrances, chances are high the students will doubt their own counting and play less confidently.