Hello Colo232 and welcome to EpicSki!
I think what you're saying is that your instructor says your shins should be gently touching the fronts of your boots, and actually what's happening is that your calves are touching the backs of your boots instead. Am I right? If so, your instructor is correct. To ski well, you need to be balanced with your knees slightly bent over the approximate center of your skis, which corresponds to having your shins gently touching the fronts of your boots. With your calves touching the back instead, your weight will be on the tails of your skis, making them harder to control. If that's really happening, you'll find your calves and back start to ache from the effort of staying in balance, and your skis feel as if they're ahead of you.
This happens to all of us from time to time, and there are three possible causes: fear, technique and your boots.
On fear: All skiers have a tendency to try to stand upright relative to the pull of gravity when things get scary. This is because our instincts say we're better off with our feet between our centre of mass and the centre of the earth. Unfortunately, once we have skis on our feet, our instincts are wrong. Your feet need to be on a line through your centre of mass on a line perpendicular to the slope, instead, since that's the only way gravity is supporting you. In the other direction, its pulling you down the slope, and if your feet are downhill of you, they'll scoot out ahead of you along with your skis, and you'll end up on your back in the snow.
On technique: Different cues work for different people, but in general you need to keep your body over your feet so your toes, knees and shoulders line up perpendicular to the snow. Many instructors will tell you to keep your shins pressed on the fronts of your boots. Others will tell you to lean forward. Personally, I prefer to think of pulling my feet back. Remember to pull the foot back at the knee and ankle, though, and not pull the whole leg back at the hip.
On boots: At your level I imagine you're probably skiing in rental boots. The first problem you may have is that the boots may well be too big for you. Ideally ski boots should be one or two sizes smaller than your regular shoes, but its hard to find rental boots that are comfortable in that size. If your boots are too big, its hard to maintain an athletic stance with your knees and ankles bent as they should be for skiing. You'll tend to stand too upright and your calves will bang against the backs of the boots.
The other problem is that the boot could be the wrong shape for your body. If it forces your legs too far forward or if you have trouble flexing your ankles with the boots on, you'll find you tend to lean back. This all gets a bit technical, but when selecting boots try to squat down as low as possible with the boots on. If you loose balance forwards or backwards, or your heels start to lift out of the boot, try different boots.
If you decide to start skiing regularly, I would strongly recommend that your first major purchase be a pair of boots. Buy them from a good bootfitter, and get them aligned to your body shape and any have any necessary customization done. There is a list of good bootfitters in the EpicSki boot forum.