I use lots of long tones and Bach chorales (same as in concert band). When they are thin in the upper register they are not using their air effectively, but using their lip muscles to push out the notes. A lot of low, long tones will help build their embouchure. Lip flexibility studies will also help a lot (lip slurs). Some of your players may also be using those pea-shooter mouthpieces, which are nice for easy high notes, but sacrifice on tone. A good all-around mouthpiece would be a Bach 3C, it all really depends on the kid, their lips, and their practice commitment. I play on a 1 1/2 C (and did back in HS too) and can still play in the upper register fine.
When I got to the high school program I taught, the kids had one of those not-so-good, marching-bandy blast-ola sounds. Marching band can sound good too. I expect the same in tone production as I do in a concert situation, the only difference being they need to project a bit more for outdoor playing (but not blast, squeeze, and push).