I am not familiar with that test; NoviceA? Is this an eventing test?
To do extended trot in a very low level test, and 15 meter circles are usually at a slightly higher lever. Hm m mm . ..;
One idea might be to do your 15 meter circle thinking of "spiral in". Do you know spiral in? That would mean keeping a firm "wall" on with your outside leg and using the outside rein againsth the neck to kind of "push" the horse inward as it goes around. I say just think spiral in, not actually spiral in because of course then you circel would not be a real cirlce. It's just an idea.
As for Serpentines, make sure that you feel your horse take one or two steps on a straigth line (between bends) before he actually changes bend. Theoretically, the change is immediate, but since you say it comes out "zig-zaggy", then trying to imagine a little bit of straigt between the changes of bend might put greater emphasis on each loop being a complete semicircle.