In my experience, kids don't give a flip about Renaissance music. When they don't care about what you're teaching, behavior problems abound. Just ask your Freshman Algebra teachers.
Start with today's music. Whatever they're listening to. Have them bring in something to play and discuss in the class. Analyze it a bit (don't go too deep into theory, you'll lose them). Then work your way backward. Show them how today's music came from the 90s, how the 90s came from the 80s, etc. Any pop music today can be traced back through 50s rock and roll to early blues. Modern rap/R&B goes back to 80s DJs, 70s R&B/soul, 50s rock and roll, early blues. Even if they're into EDM, go back through 80s disco, back to R&B, back to the blues. You can get into Tin Pan Alley and the Great American Songbook. Maybe go into a bit of jazz. You might continue going back into the Classical era and further, but I doubt you'll have time.
You can do an interesting unit on 20th century art music. Work that in somewhere. Play some John Cage and Steve Reich. Pierre Luniere. Atonal bleep-blop music. Aleatoric stuff like Pendulum Music. Have a discussion about what is and is not music. Have a similar discussion about what is and is not art, and how it overlaps. Make the kids come up with a definition. See if their definition still applies to their current music. You'd be surprised how interested kids are in discussing this.
There are plenty of textbooks out there. If you get one that's organized by time period, go from the back to the front. Trust me, the kids REALLY don't care about Gregorian Chant. Try a book called Music: A Social Experience, it's organized more by how music fits into the culture. My kids were more interested in that concept than time periods.