Be open with the teacher and ask how you can be the best help for the class. Also, positively encouraging individual students to continue working and just to try their hardest even when they are struggling.
Don't be afraid to go up to students individually and ask them if they need help. Sometimes it can be overwhelming to go up so someone but you need to push through in order to help. Also, make sure to check in with the teacher you're mentoring for frequently.
If you find yourself standing in the back of the classroom, go and sit at a table with students and make meaningful conversation.
There may be down time in class, but make sure you are talking with your teacher to ensure you are doing everything you can to help. I found that when working with students one-on-one with textbook readings, it was helpful to read it with them and trade off, but while still letting them find the answers on their own.