What to look for in student ukuleles:
Intonation - Check the instrument's intonation by first making sure the open strings are tuned to A440 tuning (music store staff can do this), then compare each open string to a fretted note on the same string. Try the major 2nd, P4, P5, and P8 intervals (2nd, 5th, 7th, and 12th frets), or see how common chords sound.
Tone Quality - Some inexpensive ukuleles sound surprisingly good, but some sound like fishing line tied over a cigar box. Look for a sufficiently full tone with good projection (it's important for students to hear themselves!)
Brand Reputation - Does the brand you've chosen seem to be a major manufacturer of ukuleles? If so, does it have a reputation for quality control? If the answer to either of these questions is "no," you may find some duds among your order or have problems with your instruments down the road.
Geared Tuners - If you have any plans of students tuning their own instrument, geared tuners are much more user-friendly for beginners. Friction-peg tuners are somewhat less easily knocked out of tune, but their 1:1 turning ratio means your touch has to be as sensitive as your ear.
Cases - Some starter ukuleles come with "carrying bags" which are thin and designed only for carrying--not protecting--the instrument. You'll want to at least get padded gig bags for any school instruments. Even if you store instruments on the wall or in a rack, it may be necessary to transport them at some point and you'll want to keep them safe.