Hola, and I'm glad you asked! You are correct, while the site is still in Beta mode, we haven't gotten all the descriptions up and easy to find just yet.
Juniors is for upper elementary through junior high. Level 1 and Level 2 refer to High School Spanish 1 and 2, just like we did 'way back' when we were in High School ;-) 8th graders can still go ahead with Level 1, but I would recommend "half speed" focusing on vocabulary one week, and grammar the next. After you've completed Flip Flop Spanish Level 2, you'll be ready for either Juniors or Level 1. :-)
You'll find confidence and rapid growth based on your earlier experiences and you'll absorb the language in a deeper way. Some will be review, but the stories and conversations from the practice and textbook will be new. Hope this helps!
Sra. Gose