Getting Middle School Students Out of Their Shell

So I signed on recently to teach an acting class to a group of kids ages 8-14. We had our first class today and it went pretty well, but one thing I noted was there was a GLARING difference between the kids in late elementary and the kids now in middle school. The younger group are completely uninhibited and with only a couple of exceptions (that I'm guessing will be alleviated the more we spend time together as a group) they had no problem and were actively excited to play a variety of theater games I had planned.

The older kids on the other hand mostly felt like they wanted to blend into the wallpaper. We went around the circle at the beginning with introductions and talks about what each person wanted to get out of the class (it'll be a two and a half month class every Saturday) and I could tell that several of the older kids genuinely wanted to learn and get better at acting, but getting them to loosen up and not be self conscious seems potentially impossible.

I'm a long time theater professional, but have very little experience working with and teaching kids. I have game plans going forward, but I just want to create an environment where these older kids can feel free to cut loose and be loud and silly and make big choices without embarrassment or at least feel comfortable speaking loud enough so that they can be heard across the room. I genuinely don't know what happens between this age range where the younger kids have NO problem projecting, but the older kids speak almost in a whisper.

I want to engage with them on a level they're excited in, and I'm not worried at all about dealing with the 8-11 crowd. Getting them focused will be the challenge but one I feel like I can deal with. But if anyone has tips for breaking these older kids out of their shells, please. I am all ears. Thanks in advance. :)