Update your app and relaunch it after updating it a popup asks you to participate and also you can find it in your feed section.
Click on join Now and complete one lesson Today (You can also complete the same lesson again) and after completing it a popup will appear if you succeeded and repeat the same 14 more days and you'll get the badge and certificate.
Also, check Hot today's question in QnA, you'll know about it.
Also, visit the links there.
BTS(Back to school) Badge