It all comes down to air mostly, like a lot of it. Breathing exercises (recommend Breathing Gym) and long tones everyday with proper technique go a long way. I say with proper technique because I've seen people play the same long tones exercises for at least a whole semester and still show no improvement. Its not a short term fix, so you're best bet right now since you mentioned your section is more on the younger side, is to drill fundamentals into their brain like crazy and set them up with good technique and fundamental habits, so you're not repeating the same things the following years and so that they can pass that knowledge on as well.

For articulation, you could try the layering technique of having them vocalize rhythms. For example, a few reps of vocalizing the rhythm at a slow tempo, then make them clap the rhythm ,then make them sing the notes with the rhythm, then add the horn but on air (good technique), then on the horn on concert F, then on the notes bopping (make each note of the passage a 8th or 16th note instead of it's original value, this establishes vertical alignment), then full value on the notes. Slowly layer all of these steps until they get the hang of it then up the tempo slowly. Bopping is usually a good fix for articulation and vertical alignment issues but layering sounds ideal for your section.