And "well regulated" did not mean "controlled and restricted" it meant "competent and well equipped". It's like saying a mechanical device is "in good regulation". It may sound slightly anachronistic to a modern ear, but the meaning is clear. ETA - posted this while Pandora was making the same point
It's also quite clear that "A well regulated militia" was merely being offered as a relevant and important example in support of the next clause, which is the real meat of the Amendment: "the right of the people to keep and bear arms shall not be infringed." What is the militia? The militia is the people themselves, in contrast to a full-time professional military. If the people themselves are not free to keep and bear arms, then they cannot function as a militia.
The school assignment here is quite clearly, and deliberately, wrong. It's nevertheless something of an improvement, in a weird way. I remember my 9th grade class on American government had a textbook that parroted the then-universal claim in academia that the 2nd Amendment only guaranteed the states could form a militia, and that we had one called the National Guard. This was 1991-92. The gun rights movement has done well since then, because the same sort of mealymouthed textbooks and school assignments are now forced to acknowledge that it applies to individuals, even if they are still trying to neuter it with a bunch of weasel words and imaginary conditions.