How and where to sign up for Back to School

Answer 1

First, Update ur app u get pop-up noticification of back to school contest or u can join from activity feed

Answer 2

You need to complete one lesson a day for 15 consecutive days to unlock the badge and to get a certificate. More frequently asked questions can be found here

https://get.sololearn.com/back-to-school/faq

Good luck!