At my old riding school it was a MUST that you had to help out atleast once a month all day or half-day if you were too young if you wanted to ride there. It was to learn the basics in horse care and once you were old enough/capable enough you could deal with the pony you were riding e.g tack/untack grooming, and it was really a matter of if you got there early enough to tack up or someone else would do it for you.
At my new riding school I waited 5 months, until one day everyone was busy so I just asked if they wanted me to untack my horse ( I already knew how to tack/untack), and now I'm allowed to do it whenever the horse isn't being ridden next.
Just ask and I'm sure they will say yes. You learn and it saves them the trouble =P