If you could memorize a mathematical concept after it is explained to you only one time, then you have a very unique ability. Odds are, however, that you cannot. No slam on you; you're just like the rest of us. This is the reason why math teachers assign 50 multiplication problems each night for homework. They know that it's not fun to do, but it will drill in the idea so that you'll be prepared for the test.
Just as this is the case with school, so it is with anything in life, particularly in learning guitar. And since this is the case, and you have had years of experience with this concept through your entire school career, then why wouldn't you apply this same concept to learning guitar. The bottom line is: don't expect that tinkering around with the material covered in a lesson for all of 15 minutes will get you close to even remembering what the lesson was about the following week. To do anything even semi-well, you have to practice!
To be honest, I could allow you to neglect guitar each week, and just roll with it, basically getting paid to sit in a room with you and do nothing. But that wouldn't be right. My job is to TEACH you how to play guitar, not magically turn you into the next Jimi Hendrix by chillin with you for 1/2 hour each week. Furthermore, you or your parents are paying good money for you to learn guitar. Don't cheat yourself. Spend time practicing. You'll be glad you did when you're melting people's faces off with incredible guitar skills!