Believe it or not, children are actually much more adept at learning language than teens/adults. A lot of people believe in this idea of "natural learning" where you learn through strictly immersion like a child does, and then copy lessons they would take in school. I have found from personal experience that this simply doesn't work well, and certainly isn't an efficient way to learn language. The only thing I would use school grades for would be to measure your level of vocabulary, in which case I would suggest attempting to start at a 3rd-5th grade vocabulary level. Hope this helps :)

Children's brains are simply different: they have innate abilities, which, I'm afraid, we lose as we grow up. However, I do think we adults can learn from children - and make learning FUN at times. We forget the value of play, of games, of quizzes, of poems and rhymes, and, of course, songs. How about getting a kiddies' book of nursery rhymes (good for pronunciation and fluency) then, after you've learnt the rhyme (very useful for when you have children of your own) you can dig into the origin of the rhyme. Researching the origins is real grown-up stuff. There are all sort of psychological and historical facets to any rhyme. Many rhymes have really nasty beginnings - The Black Death, for example. Fascinating and scary stuff.

The way we ALL learned our first language is by listening to people around us talking, and being interested in what they were saying. The learning approach that most closely matches that, is called "Comprehensible Input" - you can google that and you will find lots of ideas! It takes time, but it's easy and fun. You have to find things that you're interested in and that are not too difficult for you. Lots of reading and listening will make you effortlessly *acquire* the language. That's how you learned your first language, right? :)

Children's brains are different, as MoiraWendy says. Furthermore, I believe I've read that one's first language is actually localized in a different area of the brain than languages learned later. Learning in the same way children learn is sometimes called the "immersive method." Many years ago, the Berlitz language schools used the slogan "You've already learned one language the Berlitz way." As I struggle to learn Spanish, my own theory is to try to use as many different ways as possible... everything I can think of. A few months ago I changed the base language on my computer to Spanish, for example, so that I am always using a tiny little bit of Spanish all the time I'm at my computer.