Depends where and what sort of work. If its gentle work in the school and the feet are quite strong then I can't see a problem. I wouldn't hack out or do anything to damage the foot, and would be more inclind to work a horse that has lost a back shoe rather that a front shoe!
I was thinking about this last week when my horse lost a shoe behind. I decided against riding her and gave her a few days off till the farrier came. It would just be sods law that the one time I rode her with a shoe missing something would go wrong lol. I didn't think it was worth risking.
I once worked a horse without a front shoe. He was all over the place!! Depending on the horse I might do though, but I don't think either of us enjoyed that schooling session!
If the horse had lost a hind shoe I probably wouldn't - it depends on the horses feet but I find they tend to crack and you can make quite a mess of the foot. If it was imperative the horse was exercised then I would do gently on soft ground, but only if really necessary.
providing the foot is in good condition i continue to work my horse in the school with a missing shoe
we are dressage orientated , he works just fine
i wouldn't jump , hack etc
if however the foot is damaged i go to the other extreme , he is kept in until the farrier arrives as ultimately his soundness & ability to work in the longer term is the most important issue
my horse is cob and has brill feet but he half pulled a shoe on his hind on sunday so a friend had to take it off, the farrier is coming tomorrow to put it back on, i would of ridden him in the field if i had to wait longer for farrier or if the weather hadn't been so s**t this week!! but would never hack out as if something went wrong id never forgive myself!! im too soft on my horses!!!