Sharers are too unreliable. It's really hard to get anyone to commit to paying £XX a month for sharing your horse, which is a problem if you are reliant on that money. Working livery is fine so long as you set in advance how often the school will be using your horse, what for, and when. Keeps your horse fit!
My yard charges very little for working livery (under £100 a month) and that includes everything (care, bedding, feed.)