Work on air flow exercises like VC studies and remingtons that aren't technically difficult but require that air connect notes. Start at a mezzo forte, then progress to mezzo piano, forte, and piano - over a period of MULTIPLE classes. This isn't something you fix in a single day or even in a single month. Do this EVERY day. EVERY time you warm up, insist that they use good, solid, confident air, which may mean that they're playing louder than you want.
Learning to take that air and play soft comes later.