Treat everything as a skill that you must train them to perform. So before they even have horns in their hands, and you are teaching rhythm (foot tap included of course), ask them to listen for the "together" of the foot tap. Don't forget to reinforce that every time foot tap is audible.
When counting, concentrate on the right syllables AND the precision of their counting. (Precision can only be judged by listening.) Train them to listen for precise counting.
When they play that first note, encourage them to listen to each other and start it together at your downbeat. If it is not good (it won't be), let them hold that first note as you normally would. Make whatever suggestions you need to make for tone etc., then start them again and ask for a better (more together) start.
I imagine you get the point.
As you work through their development, "listen" must be a recurring encouragement so that they have that as a skill that they use daily.
Of course, you find different ways to approach that, such as:
Was that together?
Who started before the rest of the band?
How do we know if those eighth notes are together?
Keep asking good questions of people who know more than you.