Generally I do not allow/encourage 7&8 grade boys to sing bass. Some people would suggest they should sing soprano or alto until their voices have changed. I usually let them choose the part they want to sing, but then may have them switch if I need help in another part or if that part doesn’t fit their range.
To help them sing their parts that are more difficult I use a notation program. I prefer Noteworthy but one like MuseScore is free. It does take some time to enter all the notes to a song, but it has been beneficial to me in the absence of a piano. It can also be helpful to let them listen to recordings of the music and let them sing along. This helps establish the tone, tempo, and mood you are looking for. I can often find recordings on YouTube or Amazon.