If hormone A is part of a negative feedback loop with B, then A will be highest when B is lowest... but why?

Let's say you have way too much calcium in your blood. Hormone A (calcitonin) is the one to fix that for you b/c it tells the kidneys to excrete Ca and decrease Ca-releasing processes by your bones. Given a bit of time, you will have normalized you Ca level back to where it belongs. Hormone B (PTH) has the opposing action: it tells the kidneys to reclaim calcium for you, it tells your intestines to absorb more Ca, and it tells the bones to release Ca into your fluids. The reason hormoneB (PTH) is needed is  b/c Ca is too low in your blood. You want PTH (hormone B) to be absent when you have too much Ca (when hormone A, calcitonin, is in full force) b/c it is designed to fix the opposing scenario.

An oversimplification of this process would be to compare it to the thermostat in your home. when you turn on the heater (A) it start producing heat (B). So when the heater is crankin' at full blast is when the heat hasn't yet risen to the point when it automatically turns off the heater by way of the thermostat.

gland A continuously releases hormone A. The hormone A then travels to gland B and triggers activation of gland B to release hormone B. If too much hormone B accumulates, the hormone B will attach itself to gland A and deactivate gland A so it stops releasing hormone A. Hormone A will get used up (and now since no new hormone A is being produced), levels of hormone A will decrease over time. When the level of hormone A decreases enough, it will no longer activate gland B and gland B will no longer produce hormone B. Since hormone B levels will get used up (and now no new hormone B is being produced), the amount of hormone B will decrease over time. When the level of hormone B decreases enough, it will no longer deactivate gland A, which will now start to produce hormone A.

This creates a cycle where the production of both hormones fluctuates like a teeter totter.. increasing the weight on one side causes a "compensation" by adding weight on the other side. Most hormones do not actually fluctuate but rather they reach an equilibrium where the hormones balance each other out and remain at a stable level.

This is the simplest explanation I can give you, if you don't understand this, you need to get help from your teacher.