Thanks for reaching out. Live streaming is considered an ephemeral use so no additional license is necessary for the use of a sound recording playing during the ceremony. If the ceremony is recorded and uploaded to YouTube a streaming synchronization license is needed, but if you are only live streaming and not rebroadcasting no further license is needed.
The ephemeral nature of a live broadcast is considered a public performance, but high schools are typically exempt from public performance licenses.
If you'd like to research public performance rights I would suggest going to ascap.com, but please feel free to contact us as well with any additional questions.